test.cs

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

  byte[] bPDF;

  // Extract the completed form field values and create Claim model.
  // This data could be stored in a database at this point
  Claim completedClaim = new Claim();

  foreach (PropertyInfo propertyInfo in completedClaim.GetType().GetProperties()) {
    var dataPoint = data.FormFields.Find(i => i.Name == propertyInfo.Name);

    if (dataPoint != null && propertyInfo.PropertyType == typeof(DateTime) &&
        DateTime.TryParse(dataPoint.Value, out var parsedDate)) {

      propertyInfo.SetValue(completedClaim, parsedDate);
    }
    else if (dataPoint != null) {
      propertyInfo.SetValue(completedClaim, dataPoint.Value);
    }
  }

  // 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);
}

private void FlattenFormFields(ServerTextControl textControl) {
  int fieldCount = textControl.FormFields.Count;

  for (int i = 0; i < fieldCount; i++) {
    TextFieldCollectionBase.TextFieldEnumerator fieldEnum =
      textControl.FormFields.GetEnumerator();
    fieldEnum.MoveNext();

    FormField curField = (FormField)fieldEnum.Current;
    textControl.FormFields.Remove(curField, true);
  }
}