Hello Canyonlight,
Thanks for that info and for choosing the Apple Support Communities. If I understand correctly, you are unable to get PDF files that are in the Books app on your Mac to also appear in the Books app on your iPad. One way to do this would be to use the Finder app to sync content from your Mac to your iPad, and here is more info on this process:
Use the Finder to sync your iPhone, iPad, or iPod with your computer
Another way would be to use iCloud to sync the PDF files, and here is more info on this process:
"You can see PDFs and books that are not from the Book Store across your iPhone, iPad, iPod touch, and Mac where you’re signed in with the same Apple ID.
Go to Settings > [your name] > iCloud, turn on iCloud Drive, then turn on Books. Then go to Settings > Books, and turn on iCloud Drive."
Read PDF documents in Books on iPad
"On your Mac: Choose Apple menu > System Preferences, click Apple ID, then click iCloud. Next to iCloud Drive (make sure it’s selected), click Options, then select Books."
See your books and audiobooks on all your devices in Books on Mac
Cheers!