How can I upload books/documents on my old iPad Gen 4?

My iPad model -A1459- has served me for a very long time and it's not been updated after IOS 10.3.3 but that's fine with me. I like the screen to display my pdf texts, books and score sheets (music notes etc) because I like the display quality far better than my e-reader, which is fine for e books etc but not for the pdf format.

BUT! I can't use iTunes - which is Music.app 1.5.1.24 nowadays - anymore to download books/pdf files, although my iPad is visible in the sidebar. The books app displays the books I have in my library but doesn't show my iPad in the sidebar.

So.. Please, can somebody help me out to solve this problem and how I can upload books or documents.

Thanks in advance, friends

Hans

[Re-Titled by Moderator]