Granted, I may not completely understand your question. However, I do believe that is not an 'error' per se but the general function of saving a PDF Form. I think that you must do a 'Save As..." in order to save any information entered into a PDF form.
Alternatively, I offer the following:
using iTextSharp.text.xml;
using iTextSharp.text.pdf;
using iTextSharp.text;
...
public void XmlToPdf(string xmlDoc, string strFilename)
{
Document document = new Document();
MemoryStream os = new MemoryStream();
PdfWriter instance = PdfWriter.GetInstance(document, os);
StringReader input = new StringReader(xmlDoc);
XmlTextReader reader = new XmlTextReader(input);
ITextHandler handler = new ITextHandler(document);
try
{
handler.Parse(reader);
}
catch (Exception exception)
{
os.Close();
throw exception;
}
finally
{
reader.Close();
input.Close();
}
FileStream output = new FileStream(strFilename, FileMode.Create);
BinaryWriter writer2 = new BinaryWriter(output);
writer2.Write(os.ToArray());
writer2.Close();
output.Close();
os.Close();
}