test.cs

[HttpPost]
public string ExportPDF([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);

    FlattenFormFields(tx);

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

  return Convert.ToBase64String(bPDF);
}