This is the same idea for the other half of my studies, the handwritten notes. I take them on a convertible laptop, and I've had a hard time with apps for that, taking notes from a device to another, exporting as PDF, needing an account for many of them, etc. So I've been working on the version of it that I always wanted. Open a tab, the slides of the lecture or multiple PDFs open next to the page or even become the page, write on them with my pen, and nothing ever leaves my browser.
braiNOTES is a static website and has no backend. Strokes are perfect-freehand outlines which support pen pressure, the PDF files are rendered by pdf.js in a worker, and for storage it uses IndexedDB a moment after every change. No account, no analytics, and completely offline and local-first. You can export notebooks as .brainotes files, which in the background is a zip with one json per page + the PDFs and pictures you import. It keeps the original pages and puts the vectorized ink on top, this way the text stays selectable in the slides.
You can try it here: https://notes.swimmingbrain.dev GitHub Repo: https://github.com/swimmingbrain/brainotes (MIT)
In Study mode, the PDF opens on the side of the tab, you can read it simultaneously to the notes, snip a formula or diagrams directly into the notes with the snip tool, and then write around it. In Board mode, you have just a Whiteboard for tutoring or simply for more place for exercises, and can also highlight or draw things with the laser pointer tool while presenting.
The line of the ink appears the moment the pen moves, and a one euro filter calms shaky hands while you write + a light a refit when you lift the pen.
I also have a video editor and an animation tool which are still a work in progress, on the same GitHub profile. I'm also working on a MCP server for each tool, so you can use your MCP client to do further things with your notes, automatically edit videos on the video editor, etc. That feature will of course be off unless you turn it on and will only be able to talk to localhost.
I've used this tool for every lecture in the last few months and a few friends use it too. Bug reports, fixes, recommendations, feedback are all welcome!