test.cs

[HttpPost]
[Route("Sign")]
public string Sign([FromBody] SignatureData signatureData)
{
  byte[] bPDF;

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

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

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

    // create a list of digital signatures
    var signatureFields = new List<DigitalSignature>();

    // create a digital signature for each signature box
    foreach (SignatureBox box in signatureData.SignatureBoxes)
    {
      signatureFields.Add(new DigitalSignature(cert, null, box.Name));
    }

    // create save settings
    SaveSettings saveSettings = new SaveSettings()
    {
      CreatorApplication = "Your Application",
      SignatureFields = signatureFields.ToArray()
    };

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

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