If you want to clean up your library you can remove individual or a whole bunch of files. In the library, long press on one of the documents.
Selected documents will change their color and will be marked with a checkmark. Tap on documents to select or deselect them.
If you changed your mind, tap on "Done" and nothing will be changed in your library. Tap on the trash icon to remove the documents from the device.
Warning: There is no way to undo that. If you want these documents again you will need to import them again.