This is close to my use case as well. I mainly buy e-books these days, but they are often published as an afterthought[1] and not designed for readers – things like maths and plots get mangled in the process and vary from excellent to completely unreadable. So I want something closer to the physical book to look up the equations and visuals, but I don't want to have to go to the library[2] for each one. A PDF scan of the print book is very convenient.

[1] Sometimes they are literally an OCR of the print book! You can tell because of the symbol substitutions.

[2] Even most digital libraries I've tried are very inconvenient to "go to". Archive.org might be the exception.