PDFs in Files or Books App won’t open on my iPad Air

The pdfs or ebooks that I have on my books or files app , suddenly stops getting opened. It just shows "can't open" and nothing else. As a student, I have suffered huge loss because of this. And now, a new problem have arrived. Whatever markings I am making on the pdf is just not getting saved, which simply means I can't read any book/notes.

I have restarted my ipad several times, deleted apps and other files to crazte space(thinking it's because of shortage of space), but nothing worked!

[Re-Titled by Moderator]