Why are PDFs missing in iBooks on iPad

Why are PDFs that I add to my library (stored in iCloud) using iBooks on my iMac not appearing in my library using iBooks on my iPad?

Before I used iCloud for my library, I used iTunes. PDFs that I synced to my iPad using iTunes still appear in iBooks but not other PDFs. Before I used iCloud, all PDFs that I added using iBooks on my iMac would appear in iTunes but now when I open iTunes there are no PDFs.

I have tried:

- Deleting iBooks on my iPad and reinstalling

- Toggling iBooks in Apple ID > iCloud Drive in Settings on my iPad

- Toggling iBooks > Sync Bookmarks and Notes and Sync Collections in Settings on my iPad