test.cs

[HttpPost]
public string PDFSignSignatureFields(string document) {
   byte[] bDocument;

   // create temporary ServerTextControl
   using (TXTextControl.ServerTextControl tx = new TXTextControl.ServerTextControl()) {
      tx.Create();

      tx.Load(Convert.FromBase64String(document), BinaryStreamType.InternalUnicodeFormat);

      X509Certificate2 cert = new X509Certificate2(Server.MapPath("~/App_Data/textcontrolself.pfx"), "123");

      List<DigitalSignature> digitalSignatures = new List<DigitalSignature>();

      foreach (SignatureField field in tx.SignatureFields) {
         field.Name = Guid.NewGuid().ToString();
         digitalSignatures.Add(new DigitalSignature(cert, null, field.Name));
         field.Image = new SignatureImage(Server.MapPath("~/App_Data/Documents/signature.svg"), 4);
         field.SignerData = new SignerData("Tim Typer", "Developer", "", "", "Online Demo");
      }

      TXTextControl.SaveSettings saveSettings = new TXTextControl.SaveSettings() {
         CreatorApplication = "TX Text Control Sample Application",
         SignatureFields = digitalSignatures.ToArray()
      };

      // export to PDF
      tx.Save(out bDocument, BinaryStreamType.AdobePDF, saveSettings);
   }

   // return as Base64 encoded string
   return Convert.ToBase64String(bDocument);

}