I'm experiencing the same issue. Since concatenating xfa forms doesn't work as expected (see this forum thread), I tried to work around this issue by enabling the user rights for Adobe Reader in Adobe Acrobat for our pdfs. I don't know exactly how this menu item is called in English (I'm using the German version of Adobe Acrobat), so I attached a screenshot of the menu item I used.
However, with these new pdfs the program crashes with the error message "Aspose does not support Field flattening in append mode." when executing the following code:
PdfFileMend pdfFileMend = new PdfFileMend(inputStream, outputStream);
pdfFileMend.AddText(new FormattedText("Test"), 1, 100, 100);
pdfFileMend.Close(); //Program crashes here
I'm using the latest available version 5.4.0 of Aspose.Pdf.Kit and attached the Visual Studio project and the input pdf.