When I Dock my iPad with my Win 10 Pro Desktop PC via the USB/Lightning Dock, a Win 10 Explorer Window pops up showing the iPad as a Drive under “This PC”.
I just copied a PDF into the iTunes Library (
C:\Users\Your Name\Music\iTunes\iTunes Media\Books) on the Win 10 PC and re-named it to “Copy test for Merlin IIS.pdf”. I did a <Right Click> and “Copy” and then navigated to iTunes (with the iPad docked) and in the iPad “Books” Panel, did a <Right Click> and “Paste” and the PDF appeared.
Not sure why it works in my environment but not yours, other than some “hidden” security setting?
When I then opened iBooks on the iPad I could open the PDF and read it as normal.
I can sympathise with your problems, I think Apple have really stuffed up when they remove Books and Apps from iTunes…
Cheers
Dennis