Unable to manually add PDF files to iPhone 6

I am currently using iTunes 12.3.3.17, and I am able to manually move selected PDF files from my books collection to my iPad HD and iPad Pro. I am not able to do the same for the iPhone 6, though. Any attempt to add a PDF without syncing to the Books library fails (I have several downloaded PDFs on the phone that are not in my iTunes library which would be erased if I opted to sync, so that is not an option).

Can anyone offer some insight as to why this might be happening?