You can check a checkbox on the XDP submit format that says "Include PDF". (I can't remember the exact wording, but it's something like that.)
This will cause the entire PDF file to be base64 encoded, and embedded into the XDP XML file.
You should then be able to extract it from the XML, decode it, and save it.
Note that submitting PDf in any way from a form requires either Acrobat or Reader Extensions.
Howard
http://www.avoka.com