Digitally sign my PDF file using USB token in dot net.

To digitally sign a PDF using iTextSharp and a USB token, you will need to first install the iTextSharp library for C#

Example :

using iTextSharp.text.pdf;
using iTextSharp.text.pdf.security;
using System.Security.Cryptography.X509Certificates;

class Demo {
 
    static public void Main(String[] args)
    {
       X509Certificate2  certificate = FindUsbCertificateByName("Name")
       string pdfPath = "C:\Users\Admin\Desktop\MainPDF.pdf";
       string SignpdfPath = "C:\Users\Admin\Desktop\MainPDF_Signed.pdf";
       SignPdf(pdfPath,SignpdfPath,certificate)
    }

public X509Certificate2 FindUsbCertificateByName(string name) {
    X509Store store = new X509Store(StoreName.My, StoreLocation.CurrentUser);
    store.Open(OpenFlags.ReadOnly);
    X509Certificate2Collection certificates = store.Certificates.Find(X509FindType.FindBySubjectName, name, true);
    store.Close();

    if (certificates.Count > 0) {
        return certificates[0];
    } else {
        return null;
    }
}


public void SignPdf(string pdfPath, string signedPdfPath,X509Certificate2 certificate) {
    // Open the PDF document
    PdfReader pdfReader = new PdfReader(pdfPath);
    PdfStamper pdfStamper = PdfStamper.CreateSignature(pdfReader, new FileStream(signedPdfPath, FileMode.Create), '\0');

    // Create a signature field
    PdfSignatureAppearance signatureAppearance = pdfStamper.SignatureAppearance;
    signatureAppearance.SetVisibleSignature(new Rectangle(36, 748, 144, 780), 1, "sig");

    // Create the signature appearance
    signatureAppearance.Reason = "Approval";
    signatureAppearance.Location = "Location";
    signatureAppearance.SignatureGraphic = Image.GetInstance("signature.png");

    // Create the signature
    IExternalSignature externalSignature = new X509Certificate2Signature(certificate, "SHA-256");
    MakeSignature.SignDetached(signatureAppearance, externalSignature, new Org.BouncyCastle.X509.X509Certificate[] { certificate }, null, null, null, 0, CryptoStandard.CMS);

    // Close the PDF document
    pdfStamper.Close();
}


}

The Spire.PDF library supports signing PDFs using USB token.

https://www.e-iceblue.com/Download/download-pdf-for-net-now.html

Code sample:

//Load PDF document 
 PdfDocument doc = new PdfDocument();
doc.LoadFromFile("test.pdf");
PdfPageBase page = doc.Pages[0];
 
//Get the certificate
X509Store store = new X509Store(StoreLocation.CurrentUser);
store.Open(OpenFlags.ReadOnly);
X509Certificate2Collection certs = store.Certificates.Find(X509FindType.FindBySerialNumber, "Your USB Token Cert", false);
X509Certificate2 cert1 = certs[0];
PdfCertificate cert = new PdfCertificate(cert1);
 
Spire.Pdf.Security.PdfSignature signature = new Spire.Pdf.Security.PdfSignature(doc, page, cert, "signature");
signature.DocumentPermissions = PdfCertificationFlags.ForbidChanges;
signature.GraphicsMode = Spire.Pdf.Security.GraphicMode.SignDetail;
signature.Certificated = true;
doc.CompressionLevel = PdfCompressionLevel.Best;
 
//Set the dispay mode of graphics
signature.GraphicsMode = Spire.Pdf.Security.GraphicMode.SignImageAndSignDetail;
signature.NameLabel = "Signer:";
signature.ContactInfoLabel = "ContactInfo:";
signature.ContactInfo = signature.Certificate.GetNameInfo(X509NameType.SimpleName, true);
signature.DateLabel = "Date:";
signature.Date = DateTime.Now;
signature.LocationInfoLabel = "Location:";
signature.LocationInfo = "Chengdu";
signature.ReasonLabel = "Reason: ";
signature.Reason = "The certificate of this document";
signature.DistinguishedNameLabel = "DN: ";
signature.DistinguishedName = signature.Certificate.IssuerName.Name;
 
//Set font
signature.SignDetailsFont = new PdfFont(PdfFontFamily.TimesRoman, 10f);
signature.SignNameFont = new PdfFont(PdfFontFamily.Courier, 15);
 //Save the Pdf document
string output = "output.pdf";
doc.SaveToFile(output);

If you get problems, you can post them on the library's forum: https://www.e-iceblue.com/forum/spire-pdf-f7.html?sid=71da8c49b8ad86c39c8c81ca3df3c5b8