RichTexBox page Reader

    partial class Form1
    {
        private System.ComponentModel.IContainer components = null;

        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        private void InitializeComponent()
        {
            this.btnOpenFile = new System.Windows.Forms.Button();
            this.lblPages = new System.Windows.Forms.Label();
            this.btnPagePrev = new System.Windows.Forms.Button();
            this.btnPageFirst = new System.Windows.Forms.Button();
            this.btnPageLast = new System.Windows.Forms.Button();
            this.btnPageNext = new System.Windows.Forms.Button();
            this.SuspendLayout();
            //
            // btnOpenFile
            //
            this.btnOpenFile.BackColor = System.Drawing.Color.FromArgb(((int)(((byte)(255)))), ((int)(((byte)(128)))), ((int)(((byte)(0)))));
            this.btnOpenFile.FlatAppearance.MouseDownBackColor = System.Drawing.Color.FromArgb(((int)(((byte)(128)))), ((int)(((byte)(64)))), ((int)(((byte)(0)))));
            this.btnOpenFile.FlatAppearance.MouseOverBackColor = System.Drawing.Color.FromArgb(((int)(((byte)(255)))), ((int)(((byte)(192)))), ((int)(((byte)(128)))));
            this.btnOpenFile.FlatStyle = System.Windows.Forms.FlatStyle.Flat;
            this.btnOpenFile.Font = new System.Drawing.Font("Segoe UI", 9.75F, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
            this.btnOpenFile.ForeColor = System.Drawing.Color.Black;
            this.btnOpenFile.Location = new System.Drawing.Point(12, 12);
            this.btnOpenFile.Name = "btnOpenFile";
            this.btnOpenFile.Size = new System.Drawing.Size(122, 28);
            this.btnOpenFile.TabIndex = 2;
            this.btnOpenFile.Text = "Open Document";
            this.btnOpenFile.UseVisualStyleBackColor = false;
            this.btnOpenFile.Click += new System.EventHandler(this.btnOpenFile_Click);
            //
            // lblPages
            //
            this.lblPages.BackColor = System.Drawing.Color.Transparent;
            this.lblPages.Font = new System.Drawing.Font("Segoe UI", 12.75F, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
            this.lblPages.ForeColor = System.Drawing.Color.Black;
            this.lblPages.Location = new System.Drawing.Point(301, 16);
            this.lblPages.Name = "lblPages";
            this.lblPages.Size = new System.Drawing.Size(41, 27);
            this.lblPages.TabIndex = 4;
            this.lblPages.Text = "0";
            this.lblPages.TextAlign = System.Drawing.ContentAlignment.MiddleCenter;
            //
            // btnPagePrev
            //
            this.btnPagePrev.Enabled = false;
            this.btnPagePrev.FlatAppearance.BorderColor = System.Drawing.SystemColors.Control;
            this.btnPagePrev.FlatAppearance.BorderSize = 0;
            this.btnPagePrev.FlatAppearance.MouseDownBackColor = System.Drawing.SystemColors.Control;
            this.btnPagePrev.FlatAppearance.MouseOverBackColor = System.Drawing.SystemColors.Control;
            this.btnPagePrev.FlatStyle = System.Windows.Forms.FlatStyle.Flat;
            this.btnPagePrev.Font = new System.Drawing.Font("Segoe UI Symbol", 12F, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
            this.btnPagePrev.ForeColor = System.Drawing.Color.Black;
            this.btnPagePrev.Location = new System.Drawing.Point(275, 14);
            this.btnPagePrev.Name = "btnPagePrev";
            this.btnPagePrev.Size = new System.Drawing.Size(25, 27);
            this.btnPagePrev.TabIndex = 5;
            this.btnPagePrev.Text = "◀";
            this.btnPagePrev.TextAlign = System.Drawing.ContentAlignment.MiddleLeft;
            this.btnPagePrev.UseVisualStyleBackColor = true;
            this.btnPagePrev.Click += new System.EventHandler(this.btnPagePrev_Click);
            //
            // btnPageFirst
            //
            this.btnPageFirst.Enabled = false;
            this.btnPageFirst.FlatAppearance.BorderColor = System.Drawing.SystemColors.Control;
            this.btnPageFirst.FlatAppearance.BorderSize = 0;
            this.btnPageFirst.FlatAppearance.MouseDownBackColor = System.Drawing.SystemColors.Control;
            this.btnPageFirst.FlatAppearance.MouseOverBackColor = System.Drawing.SystemColors.Control;
            this.btnPageFirst.FlatStyle = System.Windows.Forms.FlatStyle.Flat;
            this.btnPageFirst.Font = new System.Drawing.Font("Segoe UI Symbol", 14.25F, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
            this.btnPageFirst.ForeColor = System.Drawing.Color.Black;
            this.btnPageFirst.Location = new System.Drawing.Point(244, 11);
            this.btnPageFirst.Name = "btnPageFirst";
            this.btnPageFirst.Size = new System.Drawing.Size(25, 27);
            this.btnPageFirst.TabIndex = 6;
            this.btnPageFirst.Text = "⏪";
            this.btnPageFirst.TextAlign = System.Drawing.ContentAlignment.MiddleLeft;
            this.btnPageFirst.UseVisualStyleBackColor = true;
            this.btnPageFirst.Click += new System.EventHandler(this.btnPageFirst_Click);
            //
            // btnPageLast
            //
            this.btnPageLast.Enabled = false;
            this.btnPageLast.FlatAppearance.BorderColor = System.Drawing.SystemColors.Control;
            this.btnPageLast.FlatAppearance.BorderSize = 0;
            this.btnPageLast.FlatAppearance.MouseDownBackColor = System.Drawing.SystemColors.Control;
            this.btnPageLast.FlatAppearance.MouseOverBackColor = System.Drawing.SystemColors.Control;
            this.btnPageLast.FlatStyle = System.Windows.Forms.FlatStyle.Flat;
            this.btnPageLast.Font = new System.Drawing.Font("Segoe UI Symbol", 14.25F, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
            this.btnPageLast.ForeColor = System.Drawing.Color.Black;
            this.btnPageLast.Location = new System.Drawing.Point(370, 11);
            this.btnPageLast.Name = "btnPageLast";
            this.btnPageLast.Size = new System.Drawing.Size(25, 27);
            this.btnPageLast.TabIndex = 8;
            this.btnPageLast.Text = "⏩";
            this.btnPageLast.TextAlign = System.Drawing.ContentAlignment.MiddleLeft;
            this.btnPageLast.UseVisualStyleBackColor = true;
            this.btnPageLast.Click += new System.EventHandler(this.btnPageLast_Click);
            //
            // btnPageNext
            //
            this.btnPageNext.Enabled = false;
            this.btnPageNext.FlatAppearance.BorderColor = System.Drawing.SystemColors.Control;
            this.btnPageNext.FlatAppearance.BorderSize = 0;
            this.btnPageNext.FlatAppearance.MouseDownBackColor = System.Drawing.SystemColors.Control;
            this.btnPageNext.FlatAppearance.MouseOverBackColor = System.Drawing.SystemColors.Control;
            this.btnPageNext.FlatStyle = System.Windows.Forms.FlatStyle.Flat;
            this.btnPageNext.Font = new System.Drawing.Font("Segoe UI Symbol", 12F, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
            this.btnPageNext.ForeColor = System.Drawing.Color.Black;
            this.btnPageNext.Location = new System.Drawing.Point(339, 14);
            this.btnPageNext.Name = "btnPageNext";
            this.btnPageNext.Size = new System.Drawing.Size(25, 27);
            this.btnPageNext.TabIndex = 7;
            this.btnPageNext.Text = "▶";
            this.btnPageNext.TextAlign = System.Drawing.ContentAlignment.MiddleLeft;
            this.btnPageNext.UseVisualStyleBackColor = true;
            this.btnPageNext.Click += new System.EventHandler(this.btnPageNext_Click);
            //
            // Form1
            //
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(412, 290);
            this.Controls.Add(this.btnPageLast);
            this.Controls.Add(this.btnPageNext);
            this.Controls.Add(this.btnPageFirst);
            this.Controls.Add(this.btnPagePrev);
            this.Controls.Add(this.lblPages);
            this.Controls.Add(this.btnOpenFile);
            this.Name = "Form1";
            this.StartPosition = System.Windows.Forms.FormStartPosition.CenterScreen;
            this.Text = "Form1";
            this.ResumeLayout(false);
        }

        private System.Windows.Forms.Button btnOpenFile;
        private System.Windows.Forms.Label lblPages;
        private System.Windows.Forms.Button btnPagePrev;
        private System.Windows.Forms.Button btnPageFirst;
        private System.Windows.Forms.Button btnPageLast;
        private System.Windows.Forms.Button btnPageNext;
    }

-----------------------------------------------------------------------------------------------------------------------------
    using System;
    using System.Collections.Generic;
    using System.Drawing;
    using System.Drawing.Text;
    using System.IO;
    using System.Linq;
    using System.Runtime.InteropServices;
    using System.Text;
    using System.Windows.Forms;

    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        public class RTBReader : RichTextBox
        {
            [DllImport("user32", CharSet = CharSet.Auto)]
            private static extern int SendMessage(HandleRef hWnd, int _msg, int wParam, ref PARAFORMAT2 _pf);

            [DllImport("user32", CharSet = CharSet.Auto)]
            private static extern int SendMessage(HandleRef hWnd, int _msg, int wParam, int lParam);

            public enum TextAlignment
            {
                Left = 1,
                Right,
                Center,
                Justify
            }

            private string text;

            public RTBReader()
                : this(null, Color.Black, Color.White) { }
            public RTBReader(Font Font, Color Forecolor, Color Backcolor)
            {
                this.Name = "Reader";
                this.ScrollBars = RichTextBoxScrollBars.None;
                this.SelectionAlignment = TextAlignment.Justify;
                this.Font = Font ?? new Font("Segoe UI", 10F, FontStyle.Regular, GraphicsUnit.Point);
                this.ForeColor = Forecolor;
                this.BackColor = Backcolor;
            }

            public List<string> Pages { get; private set; }
            public string DocumentText
            {
                get => this.text;
                set => this.Initialize(value);
            }

            public void ReaderPage(int Page) => Text = Pages[Page];

            private void Initialize(string Document)
            {
                if (string.IsNullOrEmpty(Document))
                {
                    this.text = string.Empty;
                    return;
                }

                this.text = Document;
                this.Pages = new List<string>();
                TextFormatFlags flags = TextFormatFlags.Top | TextFormatFlags.Left |
                                        TextFormatFlags.WordBreak | TextFormatFlags.TextBoxControl |
                                        TextFormatFlags.NoPadding;


                Size textSize = TextRenderer.MeasureText(this.text, this.Font, this.ClientSize, flags);
                int NumberOfPages = textSize.Height / this.ClientSize.Height;

                if (textSize.Height > this.Height)
                {
                    this.Text = Document;
                    this.Update();

                    //Number of shown lines
                    int FirstCharOfLastShownLine  = this.GetCharIndexFromPosition(new Point(0, this.ClientSize.Height));
                    int ShownLines = this.GetLineFromCharIndex(FirstCharOfLastShownLine);
                    int TotalLines = this.GetLineFromCharIndex(this.Text.Length - 1);


                    for (int p = 0; p < NumberOfPages; p++)
                    {
                        int FirstLineOfPage = (p * ShownLines);
                        int FirstCharOfPage = this.GetFirstCharIndexFromLine(FirstLineOfPage);

                        int FirstLineOfNextPage = (p + 1) * ShownLines;
                        FirstLineOfNextPage = (FirstLineOfNextPage > TotalLines) ? TotalLines : FirstLineOfNextPage;
                        int LastCharOfPage = (FirstLineOfNextPage < TotalLines)
                                           ? this.GetFirstCharIndexFromLine(FirstLineOfNextPage) - 1
                                           : this.Text.Length;
                        this.Pages.Add(this.Text.Substring(FirstCharOfPage, LastCharOfPage - FirstCharOfPage));
                    }
                }
                else
                {
                    this.Pages.Add(this.text);
                }
                this.Text = this.Pages.First();
            }

            private const int EM_SETEVENTMASK = 1073;
            private const int EM_GETPARAFORMAT = 1085;
            private const int EM_SETPARAFORMAT = 1095;
            private const int EM_SETTYPOGRAPHYOPTIONS = 1226;
            private const int WM_SETREDRAW = 11;
            private const int TO_ADVANCEDTYPOGRAPHY = 0x1;
            private const int PFM_ALIGNMENT = 8;
            private const int SCF_SELECTION = 1;

            [StructLayout(LayoutKind.Sequential)]
            private struct PARAFORMAT2
            {
                public int cbSize;
                public uint dwMask;
                public short wNumbering;
                public short wReserved;
                public int dxStartIndent;
                public int dxRightIndent;
                public int dxOffset;
                public short wAlignment;
                public short cTabCount;
                [MarshalAs(UnmanagedType.ByValArray, SizeConst = 32)]
                public int[] rgxTabs;
                public int dySpaceBefore;
                public int dySpaceAfter;
                public int dyLineSpacing;
                public short sStyle;
                public byte bLineSpacingRule;
                public byte bOutlineLevel;
                public short wShadingWeight;
                public short wShadingStyle;
                public short wNumberingStart;
                public short wNumberingStyle;
                public short wNumberingTab;
                public short wBorderSpace;
                public short wBorderWidth;
                public short wBorders;
            }

            public new TextAlignment SelectionAlignment
            {
                get
                {
                    PARAFORMAT2 _pf = new PARAFORMAT2();
                    _pf.cbSize = Marshal.SizeOf(_pf);

                    SendMessage(new HandleRef(this, Handle), EM_GETPARAFORMAT, SCF_SELECTION, ref _pf);

                    // If 0, defaults to TextAlignment.Left
                    if ((_pf.dwMask & PFM_ALIGNMENT) == 0) return TextAlignment.Left;
                    return (TextAlignment)_pf.wAlignment;
                }

                set
                {
                    PARAFORMAT2 _pf = new PARAFORMAT2();
                    _pf.cbSize = Marshal.SizeOf(_pf);
                    _pf.dwMask = PFM_ALIGNMENT;
                    _pf.wAlignment = (short)value;

                    SendMessage(new HandleRef(this, Handle), EM_SETPARAFORMAT, SCF_SELECTION, ref _pf);
                }
            }

            // Overrides OnHandleCreated to enable RTB advances options
            protected override void OnHandleCreated(EventArgs e)
            {
                base.OnHandleCreated(e);

                // EM_SETTYPOGRAPHYOPTIONS allows to enable RTB (RichEdit) Advanced Typography
                SendMessage(new HandleRef(this, Handle), EM_SETTYPOGRAPHYOPTIONS,
                                                         TO_ADVANCEDTYPOGRAPHY,
                                                         TO_ADVANCEDTYPOGRAPHY);
            }
        }

        RTBReader DocumentViewer = null;
        private int CurrentPage = 0;
        private int FirstPage = 0;
        private int LastPage = 0;
        private void btnOpenFile_Click(object sender, EventArgs e)
        {
            DocumentViewer = new RTBReader();
            string Document = File.ReadAllText(@"..\..\Resources\Document.txt", Encoding.Default);
            DocumentViewer.Location = new Point(12, 46);
            DocumentViewer.Size = new Size(385, 230);
            DocumentViewer.DocumentText = Document;
            this.Controls.Add(DocumentViewer);
            this.lblPages.Text = "1/" + DocumentViewer.Pages.Count.ToString();
            LastPage = DocumentViewer.Pages.Count - 1;
            btnPageNext.Enabled = true;
            btnPageLast.Enabled = true;
        }

        private void btnPageNext_Click(object sender, EventArgs e)
        {
            if (CurrentPage < LastPage)
            {
                btnPageFirst.Enabled = true;
                btnPagePrev.Enabled = true;

                CurrentPage += 1;
                DocumentViewer.ReaderPage(CurrentPage);
                this.lblPages.Text = (CurrentPage + 1).ToString() + "/" + DocumentViewer.Pages.Count.ToString();

                if (CurrentPage == LastPage)
                {
                    btnPageNext.Enabled = false;
                    btnPageLast.Enabled = false;
                }
            }
        }

        private void btnPagePrev_Click(object sender, EventArgs e)
        {
            if (CurrentPage > FirstPage)
            {
                btnPageLast.Enabled = true;
                btnPageNext.Enabled = true;

                CurrentPage -= 1;
                DocumentViewer.ReaderPage(CurrentPage);
                this.lblPages.Text = (CurrentPage + 1).ToString() + "/" + DocumentViewer.Pages.Count.ToString();
                if (CurrentPage == 0)
                {
                    btnPagePrev.Enabled = false;
                    btnPageFirst.Enabled = false;
                }
            }
        }

        private void btnPageFirst_Click(object sender, EventArgs e)
        {
            btnPagePrev.Enabled = false;
            btnPageFirst.Enabled = false;
            btnPageNext.Enabled = true;
            btnPageLast.Enabled = true;
            CurrentPage = FirstPage;
            DocumentViewer.ReaderPage(FirstPage);
            this.lblPages.Text = (CurrentPage + 1).ToString() + "/" + DocumentViewer.Pages.Count.ToString();
        }

        private void btnPageLast_Click(object sender, EventArgs e)
        {
            btnPageNext.Enabled = false;
            btnPageLast.Enabled = false;
            btnPagePrev.Enabled = true;
            btnPageFirst.Enabled = true;
            CurrentPage = LastPage;
            DocumentViewer.ReaderPage(LastPage);
            this.lblPages.Text = (CurrentPage + 1).ToString() + "/" + DocumentViewer.Pages.Count.ToString();
        }
    }