How to let a user download the page?

Hello, I am currently building an app where the users can read books for free online. I want the users to have the ability to read books offline too. So I thought of some ideas:

  1. Download the page as HTML. When the book name is clicked from a list, it checks whether the user have already download the book page details as HTML.
  2. When a user opens a book page for more details about it, the app automatically caches the page.
  3. Downloading it as a pdf but it will be accessible in the download manager on computer/mobile

I want something that is only accessible by the app only and can’t be stolen. I don’t have that expertise in these stuff. If anyone can tell me what is possible to do and maybe give me more insights about this topic, that would be greatly appreciated.

Thanks in advance!

any ideas??