PDFs added on iPad don't sync to Mac

When I add PDFs to iBooks on my iPad, they never sync back (i.e., are not added) to iBooks on my Mac. iCloud Drive is enabled. iTunes controls which books get synced from Mac to iPad, but I'm not sure what other options need to be switched on to sync from iPad to Mac.

On OS X 10.11.5 and iOS 9.3.2.