This is my pet peeve with Apple. They tell you what you need and don't listen to what you think you need. They tell you how to use your phone and don't ask what you would like to do with your phone. For years it was "you don't need a bigger phone"... until they finally jump onboard and offer things called pluses and maxes. But only after years of denial.
It took me 20 minutes last night to transfer this one file.. because who knew that pdfs could only go into "books "folder"? What's wrong with photos... or just the "files" folder? With normal USB drive functionality this would have taken 30 seconds.
I went through this a couple years ago with my iPad Pro... I wanted a large PDF, a shop manual for one of my ATVs on the device...
I couldn't just copy it over like I could on my Android device... On my Note, it was plug it in, drag it over, drop it, unplug it. 20 seconds. And to open it, Android has a file manager app, just like your PC or Macbook does, click the file, it opens it. Couldn't be any simpler...
And then there is the Apple way of doing it. Downloading and installing an app, and then fiddling with iTunes to copy it over. 15 minutes. And you are right, I could only transfer it to the Books app.
Then to use is, I would have to use the app that I copied it with, find it, click on it, and it would try to open it with the default PDF viewer app, it wouldn't give me a choice. And at the time, that app could only read files up to 50mb, which this was well over. So I had to go find the PDF, load it with the default app, it would fail and allow me to select another app to open it with, and then and only then could I open it.
The kicker then became, when I wanted to open it again at some point. There was no ability to set my preferred app, so I would have to open the first app, find it again, tell it to open it with the wrong app, wait for it to fail and then give me the option to use the app I wanted to use in the first place, etc... You had to do that EVERY TIME you wanted to open a stupid PDF.
What you could do, was import the app to my preferred viewer, but because of how Apple puts a wall around every app, you now have TWO COPIES of the PDF taking up space on your device.
It is the most archaic, insane and wasteful and counter-intuitive way to do things, IMHO. There is nothing user friendly or "it just works" about it. If it takes me this much hassle to copy and open a stupid PDF on the device, imagine me trying to explain this to my mom over the phone. LOL
Apple claims to be so easy to use, but they aren't. I think that people have been so firmly entrenched in the Apple world that they don't know what they don't know. They just chalk it up to "this is how things work" and they have no idea that the rest of the Phone/tablet world cracked this nut 10 years ago... haha