PDF files removed from iCloud

I had quite a few PDFs in my iCloud. During testing of my new iPhone these have been removed and not appearing in my Recently Deleted files on iCloud. This happened yesterday (14th October). I’d downloaded one of my PDFs for testing. Worked perfectly. I then synced the phone with iTunes (w10) and included one Audible title for testing. Again worked perfectly. I don’t listen to Audible on the phone so repeated the sync without Audiobooks. iTunes gave me the option to keep or remove books. I said Remove. Some time later I checked my Library in the Books app. Empty! Nothing local, nothing in iCloud.

What happened and how can I recover my PDFs?

Many Thanks

Sal