A very good question, i've been trying to answer that for the past 24hrs...
I can't seem to find a way to "download" pdfs from emails on my ipad, there are temporary files that you transfer from app to app but that's all you can do it seems.
My goal was for someone to be able to upload jpg, pdfs to my server via the app, but it seems everything is set up for jpgs, and pictures but not for documents...
I am not sure where to go from there...
Y.