Backing up iBooks Library to a PC

I installed iTunes on my new Windows 10 PC in order to back up my book collection stored on iBooks. My iPad is and Air 2 model running iOS 10.2.1.

I synced my iPad to the PC with the lightning cable, and chose an automatic backup.

But the only books I could back up to the PC were those I had purchased directly from Apple's iBooks. How do I back up books that purchased from other parties in .pdf format and saved to iBooks?

I'd really appreciate some help in solving this problem. The books I hope to back up are geared to DSLR photographers and are not sold by typical online bookstores.