How to fill XFA form using iText?

Code:

using (FileStream pdf = new FileStream("C:/test.pdf", FileMode.Open))
using (FileStream xml = new FileStream("C:/test.xml", FileMode.Open))
using (FileStream filledPdf = new FileStream("C:/test_f.pdf", FileMode.Create))
{
   PdfReader.unethicalreading = true;
   PdfReader pdfReader = new PdfReader(pdf);
   PdfStamper stamper = new PdfStamper(pdfReader, filledPdf);

   stamper.AcroFields.Xfa.FillXfaForm(xml);
   stamper.Close();
   pdfReader.Close();
}

This code throws no exception and everything seems to be ok, but if I open filled pdf, Adobe Reader says something like that:

This document enabled extended features. This document was changed since it was created and using extended features isn't possible anymore.

Some fields are filled properly, but I can't edit it. Some fields are empty. If I choose xml manually by clicking 'Import data' from Adobe Reader, form is filled properly, so I guess there is no error in xml.

You are not creating the PdfStamper object correctly. Use:

PdfStamper stamper = new PdfStamper(pdfReader, filledPdf, '\0', true)

In your code, you are not using PdfStamper in append mode. This means that iText will reorganize the different objects in your PDF. Usually that isn't a problem.

However: your PDF is Reader-enabled, which means that your PDF is digitally signed using a private key owned by Adobe. By reorganizing the objects inside the PDF, that signature is broken. This is made clear by the message you already mentioned:

This document enabled extended features. This document was changed since it was created and using extended features isn't possible anymore.

You changed the document in a way that isn't allowed (see section 8.7 of my book entitled "Preserving the usage rights of Reader-enabled forms").

To avoid breaking the signature, you need to use PdfStamper in append mode. Instead of reorganizing the original content, iText will now keep the original file intact and append new content after the end of the original file.