May I know from where you are trying to import documents to your iPhone. Is it Document Cloud or iCloud?
Please find the details below for managing files from different sources.
For Importing documents using iTunes:
Using Adobe Document Cloud:
Open Adobe Reader on you iPhone.
Tap on Home Icon at the top left of the screen to bring tool set.
Tap on My Account and login with your Adobe ID and tap on Done
From storage option drop down tap on Document Cloud and open PDF
Let me know if this works.