I know hypothes.is supports PDFs, but their annotations work on selected text ranges and that doesn't work very well for math stuff (hard to select). The main

I know hypothes.is supports PDFs, but their annotations work on selected text ranges and that doesn't work very well for math stuff (hard to select). The main page has recently listed arXiv articles right now, we'll add the recently annotated articles soon. As for PDF.js, it's pretty well written and easy to understand. I didn't have trouble make modifications at all.