Books automatically deletes my books when offline, and when there is enough space on device There is something strange going on with Books on iPad and iPhone. It keeps deleting some books locally, keeping them in the cloud.

It's frustrating because it happens when I'm offline and I simply lose the access to them.

Why is this happening at all? And how can I stop it?

I enabled syncing things over iCloud Drive and everything, but I can't see an option to disable such optimizations.



PS: the same thing happens with Photos, Numbers and Pages. The Photos app has an optimize storage option. But there doesn’t seem to be such an option for Books, Numbers and Pages. 

A clear example: just a few hours I was reading a book on Books, I took a nap, and when I opened the Books app, the book was not available offline anymore. Since I don’t have internet right now, it’s simply impossible to access the document.
I feel you. I've never been able to figure this out, and it is so frustrating. This might not be the answer you're looking for, however, I've migrated away from using iBooks and using iCloud for syncing anything besides things like the keychain and settings.

Since you mentioned you were reading a pdf ebook, I recommend an app called Bluefire Reader. It does ePub, PDF, and Adobe DRM. Of course, if you have a bunch of ebooks you've bought from Apple, well, that's another thing; DeDRM does exist, but that's really all I can say on that.