Only way would be to transfer data between apps
For example, iBooks (app) can read PDF files, and so does Safari
If you open a PDF document in Safari, you can tap it once to show the menu (I think it's at the top right corner), then tap on OPEN IN, then select iBooks.
This would send a copy (save as) of that PDF document in iBooks.
This method would work in many many types of apps, you could transfer files like videos, or images or documents.
I would suggest the app iFiles
It has various ways of transferring files into the app and it could do basic file management such as renaming, duplicating, simple editing (TXT), uploading and downloading from many sources such as WiFi transfer, dropbox, google docs, bluetooth transfer, winzip, FB, etc
I've been using it for years and its pretty amazing