Uploading files is done through the "hamburger" icon on the upper left corner of the app screen. Click on it and you'll get the "Import Files" down on the left side. If you click on it one gets (well, I do) two choices, iCloud Drive and Dropbox. I use Dropbox, since all my files reside in Dropbox, and there it is easy to click on any cdf, which will bring it up to the iPad. Other files, you can get into the cloud somehow (no need to have Mathematica there) so that the app on the iPad can recognize them, provided both the iPad and the PC are synchronized. My technical knowledge is rather limited, but that's the general idea.