Can I modify a pdf file containing a signature field prior to download?

I have a pdf file I use as a template and in my .net application I fill in a few fields using pdfstamper, iTextsharp, etc.  I'm new to this.

That all works.

I also have a signature field in my document.

After I modify the fields, with the person's name, etc., I download it to the client machine, open it in Adobe Reader X, and ask the user to sign the document.

I get an error saying "This document enabled extended features in Adobe Reader. The doc hsa been changed since it was created and the extended features are no longer available......"

Is there a way for me to do what I'm trying to do?  I hope this is somewhat clear.  It appears that modifying the fields with itextsharp is what's causing the error to occur.