This is not a mission-critical feature to me by a long shot (as opposed to Apple Pencil inking support), but in the native PDF reader on the iOS app I'd like the ability to insert a blank page wherever I want in the PDF.

I often read papers where the authors skim over a longer calculation or proof, and I'm left to fill in the details on my own. Sometimes the margins of the paper suffice, but sometimes the details require a page or two to fully work out.

Right now I maintain Goodnotes notebooks for any papers I read that require space on a page to work through. It'd be helpful while reviewing papers I've already read to have all my notes on the paper stored inside the paper itself, without my having to manage a set of notebooks or add additional attachments to Zotero entries (which won't automatically link back to the relevant page/section in the paper).