Program.cs

using Syncfusion.Pdf.Parsing;
using Syncfusion.Pdf.Security;
using Syncfusion.Drawing;
using System.Security.Cryptography.X509Certificates;
using System.Security.Cryptography;

namespace Externally_sign_the_PDF_document {
    internal class Program {
        static void Main(string[] args) {
            //Get the stream from the document
            FileStream documentStream = new FileStream(Path.GetFullPath(@"Data/Barcode.pdf"), FileMode.Open, FileAccess.Read);
            //Load the existing PDF document
            PdfLoadedDocument loadedDocument = new PdfLoadedDocument(documentStream);

            //Creates a digital signature.
            PdfSignature signature = new PdfSignature(loadedDocument, loadedDocument.Pages[0], null, "Signature");
            //Sets the signature information.
            signature.Bounds = new RectangleF(new PointF(0, 0), new SizeF(100, 30));
            signature.Settings.CryptographicStandard = CryptographicStandard.CADES;
            signature.Settings.DigestAlgorithm = DigestAlgorithm.SHA1;

            //Create an external signer.
            IPdfExternalSigner externalSignature = new ExternalSigner("SHA1");

            //Add public certificates.
            List<X509Certificate2> certificates = new List<X509Certificate2>();
            certificates.Add(new X509Certificate2(new X509Certificate2(Path.GetFullPath(@"Data/PDF.pfx"), "password123")));
            signature.AddExternalSigner(externalSignature, certificates, null);

            //Create file stream.
            using (FileStream outputFileStream = new FileStream(Path.GetFullPath(@"Output/Output.pdf"), FileMode.Create, FileAccess.ReadWrite)) {
                //Save the PDF document to file stream.
                loadedDocument.Save(outputFileStream);
            }
            //Close the document.
            loadedDocument.Close(true);
        }
        //Create the external signer class and sign the document hash.
        class ExternalSigner : IPdfExternalSigner {
            private string _hashAlgorithm;
            public string HashAlgorithm {
                get { return _hashAlgorithm; }
            }

            public ExternalSigner(string hashAlgorithm) {
                _hashAlgorithm = hashAlgorithm;
            }
            public byte[] Sign(byte[] message, out byte[] timeStampResponse) {
                timeStampResponse = null;
                X509Certificate2 digitalID = new X509Certificate2(new X509Certificate2(Path.GetFullPath(@"Data/PDF.pfx"), "password123"));
                if (digitalID.PrivateKey is System.Security.Cryptography.RSACryptoServiceProvider) {
                    System.Security.Cryptography.RSACryptoServiceProvider rsa = (System.Security.Cryptography.RSACryptoServiceProvider)digitalID.PrivateKey;
                    return rsa.SignData(message, HashAlgorithm);
                }
                else if (digitalID.PrivateKey is RSACng) {
                    RSACng rsa = (RSACng)digitalID.PrivateKey;
                    return rsa.SignData(message, System.Security.Cryptography.HashAlgorithmName.SHA1, RSASignaturePadding.Pkcs1);
                }
                else if (digitalID.PrivateKey is System.Security.Cryptography.RSAOpenSsl)
                {
                    System.Security.Cryptography.RSAOpenSsl rsa = (System.Security.Cryptography.RSAOpenSsl)digitalID.PrivateKey;
                    return rsa.SignData(message, System.Security.Cryptography.HashAlgorithmName.SHA1, RSASignaturePadding.Pkcs1);
                }
                return null;
            }
        }
    }
}