I use everyday the Zotero IOS app on my IPad. I think that can be useful to open more that one document at times and switch from each other thanks to a tab system like a classic browser web. Another feature that can be useful is the possibility to introduce a pop-up note: I mean that inserting a note-like symbol and pressing on it opens a blank page on which you can write with the Apple Pencil(like a classic notes app).