[help] Itext sign prbolem

Dear all,	I encountered a problem when I tried to sign a signature field in a pdf file with itext.	Here is the code:	 IExternalSignature externalSignature = new X509Certificate2Signature(sign_cert, hashAlg);	 pdfReader = new PdfReader(pdf_src_filename);	 signedPdf = new FileStream(pdf_dest_filename, FileMode.Create);	 pdfStamper = 		iTextSharp.text.pdf.PdfStamper.CreateSignature(pdfReader, signedPdf, '\0', null, true);
	After my signature, the pdf file show a message box, it said:	The document has enable the Adobe Reader extension function, and it has been changed since it be create.please contact the owner to get original version of the document.		Would you please give me some hint or help to solve this problem.  	Thank You very much!

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