Here is C# code that demonstrates how to add invisible digital signature to a PDF document: 
using System.Security.Cryptography.X509Certificates;
using Vintasoft.Imaging.Pdf;

/// <summary>
/// Adds invisible digital signature to a PDF page.
/// </summary>
public class PdfPageDigitalSignatureHelper_InvisibleAppearance
{
    public static void Run()
    {
        // get the X509 certificate from .pfx-file
        using (X509Certificate2 certificate = new X509Certificate2("TestCertificate.pfx", "test"))
        {
            // create a helper that allows to add digital signature to a PDF document
            PdfPageDigitalSignatureHelper digitalSignatureHelper = new PdfPageDigitalSignatureHelper(certificate);

            // set the reason for signing
            digitalSignatureHelper.SigningReason = "Approve document";

            // add digital signature to the first PDF page, sign PDF document and save PDF document to the output file
            digitalSignatureHelper.SignDocument("TestDocumentToSign.pdf", "TestDocumentToSign-signed.pdf", 0);
        }
    }
}