////Con el método de abajo abro el almacén de certificados y selecciono uno para firmar.

private void btnExplorar_Click(object sender, EventArgs e)
    {
        X509Store store= new X509Store(StoreName.My,StoreLocation.CurrentUser);


        store.Open(OpenFlags.ReadOnly);

        X509Certificate2Collection collection = (X509Certificate2Collection)store.Certificates;
        X509Certificate2Collection fcollection = (X509Certificate2Collection)collection.Find(X509FindType.FindByTimeValid, DateTime.Now, false);

        X509Certificate2Collection scollection = X509Certificate2UI.SelectFromCollection(fcollection, "Lista de Certificados digitales instalados en su equipo", "Seleccion el certificado con el que desea firmar", X509SelectionFlag.SingleSelection);
        //  X509Certificate2 certificadoElegido = scollection[0];

        if (scollection.Count > 0)
        {

          certificadoElegido = scollection[0];//certificadoElegido es vble de clase

    }

//Con este método recorro un listbox con la lista de todos los PDF seleccionados para firmar e intento crear un campo, pero no sé como lograr que ese campo se cree en el PDF y hacer la firma. Necesito por supuesto que esa firma se vea luego en el PDF para que los solicitantes sepan quién se los firmó.

private void button1_Click(object sender, EventArgs e)
    {
        if (cLBoxListadePdf.CheckedItems.Count!=0) {
            string pdfAFirmar;
            int i;
            for (i=0;i<=cLBoxListadePdf.CheckedItems.Count-1;i++) {
                pdfAFirmar = cLBoxListadePdf.CheckedItems[i].ToString();

                PdfReader pdf = new PdfReader(pdfAFirmar);
                PdfStamper stamper = PdfStamper.CreateSignature(pdf,new FileStream("C:\Users...", FileMode.Create), '');
                PdfSignatureAppearance sap = stamper.SignatureAppearance;
                sap.Reason = "";
                //sap.Location=100;
                stamper.Close();
            }
        }
    }

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Collections;
using System.IO;
using System.Security.Cryptography;
using System.Security.Cryptography.Pkcs;
using iTextSharp.text;
using iTextSharp.text.pdf;
using iTextSharp.text.pdf.security;

using Org.BouncyCastle.X509;

using SysX509 = System.Security.Cryptography.X509Certificates;


namespace FirmaPDF
{
    public class firma
    {

        /// <summary>
        /// Firma un documento PDF
        /// </summary>
        /// <param name="Source">Path del PDF a firmar</param>
        /// <param name="Target">Path del PDF firmado</param>
        /// <param name="Certificate">Certificado para realizar la firma</param>
        /// <param name="Reason">Motivo</param>
        /// <param name="Location">Ubicación</param>
        /// <param name="AddVisibleSign">Indica si la firma es visible dentro del documento</param>
        /// <param name="AddTimeStamp">Indica si se va a añadir sello de tiempo en el documento</param>
        /// <param name="strTSA">TSA del sello de tiempo</param>

        public static void SignHashed(string Source, string Target, SysX509.X509Certificate2 Certificate, string Reason, string Location, bool AddVisibleSign, bool AddTimeStamp, string strTSA)
        {
            X509CertificateParser objCP = new X509CertificateParser();
            X509Certificate[] objChain = new X509Certificate[] { objCP.ReadCertificate(Certificate.RawData) };

            IList<ICrlClient> crlList = new List<ICrlClient>();
            crlList.Add(new CrlClientOnline(objChain));

            PdfReader objReader = new PdfReader(Source);
            PdfStamper objStamper = PdfStamper.CreateSignature(objReader, new FileStream(Target, FileMode.Create), '',null,true);

            // Creamos la apariencia
            PdfSignatureAppearance signatureAppearance = objStamper.SignatureAppearance;
            signatureAppearance.Reason = Reason;
            signatureAppearance.Location = Location;

            // Si está la firma visible:
            if (AddVisibleSign)
                signatureAppearance.SetVisibleSignature(new Rectangle(100, 100, 300, 200), 1, null); //signatureAppearance.SetVisibleSignature(new Rectangle(100, 100, 250, 150), objReader.NumberOfPages, "Signature");

            ITSAClient tsaClient = null;
            IOcspClient ocspClient = null;

            // Si se ha añadido el sello de tiempo
            if (AddTimeStamp){
                ocspClient = new OcspClientBouncyCastle();
                tsaClient = new TSAClientBouncyCastle(strTSA);
            }

            // Creating the signature
            IExternalSignature externalSignature = new X509Certificate2Signature(Certificate, "SHA-1");
            MakeSignature.SignDetached(signatureAppearance, externalSignature, objChain, crlList, ocspClient, tsaClient, 0, CryptoStandard.CMS);

            if (objReader != null)
                objReader.Close();
            if (objStamper != null)
                objStamper.Close();
        }
    }
}