I liked how this textbook website is set up. The left menu bar shows when I select a subject, study level and textbook name. When I click the Open button on the left menu, the chapter pdf shows up.
I want to make something similar for my website, which I am making on WordPress. Could you suggest me tools/ software/ plugins or any help at all on how I can do this in my website? I am directionless right now, and I would appreciate it if you could tell me what this system is called. I can do more research if I have a starting point.