Delete file from iCloud but not from Books.

I have a number of e-books and PDFs that I've downloaded to Books on my iMac, and have put the same files on my iPad using iCloud. My free iCloud is now using more than half its storage. I'd like to delete the files from ONLY iCloud (the one place I don't need them!). I can find information on how to delete my files everywhere, but that's not what I want. How do I delete from only my iCloud account, while keeping the files on my iPad and iMac?