How to get signature Value from signed PDF

How is it possible to get signature Value from signed PDF file? I can get all other data from signature except its value. Is there any way to get it in C#?

 PdfPKCS7 pk;
PdfReader reader = new PdfReader(PdfFilename);
AcroFields af = reader.AcroFields;

var names = af.GetSignatureNames();
foreach (string name in names)
{
  pk = af.VerifySignature(name);

  var CN_signer= iTextSharp.text.pdf.security.CertificateInfo.GetSubjectFields(pk.SigningCertificate).GetField("CN");
  var C_signer = iTextSharp.text.pdf.security.CertificateInfo.GetSubjectFields(pk.SigningCertificate).GetField("C");
  var CN_issuer = iTextSharp.text.pdf.security.CertificateInfo.GetIssuerFields(pk.SigningCertificate).GetField("CN");
  var OU_issuer = iTextSharp.text.pdf.security.CertificateInfo.GetIssuerFields(pk.SigningCertificate).GetField("OU");
  var O_issuer= iTextSharp.text.pdf.security.CertificateInfo.GetIssuerFields(pk.SigningCertificate).GetField("O");
  var C_issuer = iTextSharp.text.pdf.security.CertificateInfo.GetIssuerFields(pk.SigningCertificate).GetField("C");
  var nr_serial = pk.SigningCertificate.SerialNumber;
  var date = pk.SignDate.ToString();