iBooks not syncing across devices?
I spent a lot of time following various bits of advice on here and YouTube to solve the problem of PDFs not syncing from Books on my iPad to my Mac and iPhone and viceversa. I was taken down lots of rabbit holes and complications, but the answer which I discovered for myself was blazingly obvious: go into Settings on your devices and scroll down to Books and make sure these items are toggled on: Background App Refresh; iCloud Drive; and Include PDFs.
iPad Air, iPadOS 17