How do I download a file from a Mac to my iPad Mini? using iTunes?

I do a lot of reading on my iPad mini, usually by buying or borrowing Kindle books, which is pretty trouble-free. But I also get ebooks via DropBox which are readable via a Nook app on the iPad, IF I can get them into the iPad in the first place. I recall transferring books in the past using iTunes, plugging in the iPad (IOS 12.4.8) via USB, then matching apps on the iPad to files on the Mac Pro (Mojave 10.14.6), and dragging them onto the iPad. But that doesn't seem to be a simple process anymore. How should I do it?