Downloaded PDFs disappear from iPad.

I have pdfs downloaded to the Books app on my iPad Air 2. They disappear, even while I'm in the middle of reading them. They are backed up in iCloud, and that doesn't even repopulate the Books app. It is horrifically annoying. It happens all the time. Then after a few days, I'll get them back, or they'll come back from wherever they're hiding in the hard drive. Then it will be okay for a few days before it happens again. This is terrible. All the purchased books are there. They never go away. Just the pdfs.