LibrarieClass

using Microsoft.Office.Interop.Excel;
using System;
using System.IO;

namespace MultipleExcelToPDF
{
    public class LibrarieClass
    {
        private static Application excelApplication = null;
        private static Workbook excelWorkBook = null;
        private static object paramMissing = Type.Missing;

        public static int SearchDirectoryTree(string path, out string[] XLSfiles)
        {
            XLSfiles = Directory.GetFiles(path, "*.xls", SearchOption.AllDirectories);
            return XLSfiles.Length;
        }

        public static void ProcessFiles(string[] XLSfiles)
        {
            excelApplication = new Application();
            foreach (string filePath in XLSfiles)
            {

                string paramSourceBookPath = filePath;
                // Get Extension of filePath
                string extension = Path.GetExtension(filePath);
                string paramExportFilePath = filePath.Replace(extension, ".pdf"); // Replace Extension .xls or .xlsx
                XlFixedFormatType paramExportFormat = XlFixedFormatType.xlTypePDF;
                XlFixedFormatQuality paramExportQuality = XlFixedFormatQuality.xlQualityStandard;
                bool paramOpenAfterPublish = false; // not open the pdf file after publish
                bool paramIncludeDocProps = true;
                bool paramIgnorePrintAreas = true;
                object paramFromPage = Type.Missing;
                object paramToPage = Type.Missing;

                // Open the source workbook
                excelWorkBook = excelApplication.Workbooks.Open(paramSourceBookPath,
                        paramMissing, paramMissing, paramMissing, paramMissing,
                        paramMissing, paramMissing, paramMissing, paramMissing,
                        paramMissing, paramMissing, paramMissing, paramMissing,
                        paramMissing, paramMissing);

                // Save it in the target format
                if (excelWorkBook != null)
                    excelWorkBook.ExportAsFixedFormat(paramExportFormat,
                        paramExportFilePath, paramExportQuality,
                        paramIncludeDocProps, paramIgnorePrintAreas, paramFromPage,
                        paramToPage, paramOpenAfterPublish,
                        paramMissing);
                CloseWorkBook();
            }
            QuitExcel();
        }

        public static void CloseWorkBook()
        {
            // Close the workbook object
            if (excelWorkBook != null)
            {
                excelWorkBook.Close(false, paramMissing, paramMissing);
                excelWorkBook = null;
            }
        }

        public static void QuitExcel()
        {
            // Quit Excel and release the ApplicationClass object
            if (excelApplication != null)
            {
                excelApplication.Quit();
                excelApplication = null;
            }

            GC.Collect();
            GC.WaitForPendingFinalizers();
        }

        public static int ContextualCall(string folderPath)
        {
            int count = 0;
            try
            {
                count = SearchDirectoryTree(folderPath, out string[] XLSFiles);
                ProcessFiles(XLSFiles);
            }
            catch
            {
                CloseWorkBook();
                QuitExcel();
            }
            return count;
        }
    }
}