it gets worse. There is no way to share the same document with more than one iPad app. For example, a Word document imported to Pages cannot be accessed by any other iPad app, even other apps that can open Word documents. The only work-around is to import the document twice, once for each app. Further, if you ever delete an app from your iPad, any documents stored with that app are deleted as well. Thus, to make sure your documents are preserved, copy them to your Mac before deleting the relevant iPad app. Shared files are included in a Sync backup of your iPad (at least that what this Apple support article claims), but they are not accessible for a restore of specific files.
Just wow.