Sending ePubs from PC to iOS iBooks via iCloud Drive?
iCloud Drive and iBooks aren't directly linked, so you have to open Files app on your iOS device (iOS 11+) and open it in iBooks. The iBooks iCloud setting specifically refers to keeping iOS and Mac iBooks apps in sync with each other.

iBooks has a setting at the top that says "All Books" with a down triangle on it or something similar. Tap that and at the bottom of the screen, there is a toggle that says "Hide iCloud Books". Toggle that off and tap done.

Now, navigate in Files app on iOS and find the non-protected .epub you want to move to iBooks and tap it. It should download and open in iBooks.

Do note that since they aren't linked, deleting files in iBooks and deleting files in iCloud Drive should affect each other.