iBooks won't sync PDF's to iCloud properly

I've got a Mac running the latest 10.11.4 with the latest iBooks (1.5). iBooks has a bunch of PDF's of books in there that I've manually added. I'm trying to get these to sync to iCloud, with no success.

I went into Sys Prefs > iCloud and enabled iCloud Drive. When I did this, instantly, all my PDF's disappeared from iBooks! (I was able to restore them by manually finding their location in Library > Containers and re-adding them).

I do have plenty of iCloud storage space available. (I'm paying monthly for the storage and Sys Prefs > iCloud shows that I have 18 gigs free.)

I then went into iBooks and clicked on List so I can see all the PDF's at once. I enabled visibility for the iCloud column, so I could see which were synced to iCloud. At first only one (out of 50) were synced. Then two. Then zero. And now it's stuck at zero.

I did make sure, in iBooks > Store, that I have "Authorized this computer."

Nothing I do will force the PDF's to sync to iCloud.

What can I do?