How to Sync Non-iBooks Epubs and PDF from iPad Books to Mac books

Hello,

I have plenty of Epub and pdfs in my iPad Books.app. All legally acquired, not all from Apple.

I have no problem syncing my apple purchases to my Mac - but I cannot sync the non Apple ones. Neither can I sync the PDFs.

Both my iPad and mac are with the latest version (ioS11 and Big Sur) with all updates applied.

Both have all options for iCloud on (I checked so every thing I could find is on) and I have plenty of space on iCloud.

and of course both iPad and mac are connected to the same apple store id (which is different from my iCloud id)

I am just at lost of where I should be looking. I have the sync for iCloud drive ON for the PDFs (on the iPad) but cannot find where on the drive these are ! or is it something I should do manually 1 by 1 ?

Any idea of what option I am missing ? thanks