Hey, hyperpaper creator here! One of my long term dreams for the project is to make those pdfs usable as an input source for PKMs like Obsidian. I love the promise of full-featured PKMs (searchability, long-term retention) but I don't want to be on a laptop for hours curating and typing into them. An eInk tablet is my preferred note-taking and thinking device.
The idea I want to eventually pursue is set up a service where you could sync your pdf (or send specific pages via email), have them OCRed (including dates and titles specifically since they already live in well-defined parts of the pages), and then ingest them into your PKM for long-term storage and recall
Unfortunately Obsidian Cloud doesn't seem to have an API to support this at present, but I think it's theoretically feasible with Notion at least. Let me know if you build something like this :)