Transfer PDFs from a Win PC to iBooks on iPad3

I have a Win PC and an older Ipad3 (iOS 9.3.6) which I want to use as a PDF reader. Right now I copy the PDFs from my PC to iDrive/iCloud. On the iPad, in the iCloud app I "Share/Send" the files to iBooks (I think it's called "Books" now, but I have the older version). I go through transfer times: 1) PC to iDrive, 2) "Send to iBooks" on the Ipad. Since both devices are in my home, I want to eliminate these transfer times. I found some reader apps for my iPad which let me transfer PDFs by typing a URL on my PC browser. But these book apps aren't as good as iBooks (no double tap to zoom in on just the printed area, no tap on the right to flip page). I also tried connecting via the 30-pin dock to USB drive, but the iPad doesn't see the drive. My questions: 1) Where are the files I see in the iBooks app on my iPad (with "Categories" etc) actually stored? Are they on my iPad? In the cloud? 2) Is there any way to directly transfer PDFs from my PC to that iBooks storage? 3) Is there another reader app on the iPad that has these 2 features, like iBooks: a) double-tap to zoom in on just the printed area, b) tap on the right to flip page?