When I’m reading multiple notes, PDFs on a subject, I would like to keep track of it.
Eg: I may be reading Mathematics in the morning and Chemistry in the evening and Physics next day.

Each time I switch subject, I have to reopen all the documents. Or search for what I need and open or keep them all open.

Instead I would like to have all notes open on Physics when reading Physics.

Tagging is not what I’m looking for, but a list of documents that I’m currently reading