Not able to save manually edited data after filling pdf using iTextSharp - asp.net-mvc

Question

I succeeded filling out a PDF form with database data using the iTextSharp DLL. But my code breaks Adobe's extended features. Once I've filled forms using iTextSharp, the resulting document is a flat form and we can't fill it out manually again.
I already resolved the flattening problem using the following line of code.
pdfStamper.FormFlattening = false;
Now when I open the PDF file with the db data using following code, I am able to edit the form manually:
public ActionResult ViewFile()
{
string fileName = "I9 Form.pdf";
string filenames = string.Concat(Guid.NewGuid().ToString(), ".pdf");
PdfReader pdfReader = new PdfReader(Server.MapPath(String.Format
("~/App_Data/TempletePDF/") + fileName));
MemoryStream stream = new MemoryStream();
PdfStamper pdfStamper = new PdfStamper(pdfReader, stream);
AcroFields formFields = pdfStamper.AcroFields;
formFields.SetField("LastName", "John");
pdfStamper.FormFlattening = false;
pdfStamper.Writer.CloseStream = false;
pdfStamper.Close();
byte[] file = stream.ToArray();
MemoryStream output = new MemoryStream();
output.Write(file, 0, file.Length);
output.Position = 0;
HttpContext.Response.AddHeader
("content-disposition", "inline; filename=form.pdf");
// Return the output stream
return File(output, "application/pdf");
}
I am able to print the file with manually entered data using the pdf print button, but I'm no longer able to save the file with manually entered data.
When i am trying to open this saved file normally. It gives me the following error message:
"This document enabled extended features in Adobe Acrobat Reader DC. The
document has been changed since it was created and use of extended features
is no longer available. Please contact the author for the original version
of this document."

Answer 1

It sounds as if you're filling out a Reader-enabled form. In the comments, I referred to the concept of Reader-enabling:
Can I create a Reader-enabled PDF using iText? (The answer is: no, of course not!)
How can I create a Reader enabled PDF that can be signed in Adobe Reader? (The answer is: this can only be done with Adobe software.)
From these answers, you know that Reader-enabling is achieved by introducing a digital signature that uses a private key owned by Adobe.
You fill out the form using a PdfStamper that is created like this:
PdfStamper pdfStamper = new PdfStamper(pdfReader, stream);
This alters the file and breaks the digital signature. As a result, the Reader-enabling is lost and if usage rights are defined (such as saving the file manually), then these usage rights are no longer valid.
You can work around this by creating the PdfStamper in append mode:
PdfStamper stamper = new PdfStamper(pdfReader, stream, '\0', true);
Now the original file (the bytes that are signed using Adobe's private key) remain unaltered. You just add some extra bytes. This will preserve Reader-enabling.