Windows Phone does not currently support print function, nor there is any place in the phone's file system that you can access.
To print something, a third party application would have to have access to other places on the device where things you might want to print are stored, like the Office hub, and maybe pdf readers. That access does not exist for the third party developers, with one exception, the picture store. Third party programmers are allowed to access the camera roll and such. That also explains why the available print apps let you only print photos.
There are third party applications in the Store that will allow you to upload documents someplace and then print it from there. Use search terms in Store to locate them.
If you would like to see this support added, you can go to the Windows Phone Feature Suggestions website and make requests for future releases.