test.cs

[HttpPost]
public string SignDocument([FromBody] TXTextControl.Web.MVC.DocumentViewer.Models.SignatureData data) {

  byte[] bPDF;

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

    // load the document
    tx.Load(Convert.FromBase64String(data.SignedDocument.Document),
      TXTextControl.BinaryStreamType.InternalUnicodeFormat);

    // create a certificate
    X509Certificate2 cert = new X509Certificate2("App_Data/textcontrolself.pfx", "123");

    // assign the certificate to the signature fields
    TXTextControl.SaveSettings saveSettings = new TXTextControl.SaveSettings() {
      CreatorApplication = "TX Text Control Sample Application",
      SignatureFields = new DigitalSignature[] {
        new TXTextControl.DigitalSignature(cert, null, "txsign")
      }
    };

    // save the document as PDF
    tx.Save(out bPDF, TXTextControl.BinaryStreamType.AdobePDFA, saveSettings);
  }

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