Local stored PDF’s disappeared from iBooks after iOS 26 update

All of my locally stored PDF’s in iBooks disappeared from my mini iPad 5 after the iOS 26.2.1 update. Why would an update erase my locally stored iBooks? How do I get them back? I have tried shutting down and restarting my iPad but that did not fix the problem.

Help. I want my files back. This is not the first time this has happened, but the one time before that it happened restarting my iPad fixed it. This time that did not work.

I need to know how to restore my iBooks.