Hypothesis https://web.hypothes.is/ is pretty good at keeping notes on PDF. Normally it anchors the annotations for any url but I believe that for PDF is also doing some extra checksum magic to uniquely identify the PDF and apply the annotations. Furthermore you can have collaboration features such as group annotations. Useful for classes or science labs... - Source: Hacker News / 20 days ago

Tools like https://web.hypothes.is exist and have a decent number of installs. The hard part of a generic third-party commenting tool is creating the right social context for it to actually be useful. Hypothesis for example is mostly used via its integration into online learning platforms, where that context already exists. - Source: Hacker News / 3 months ago

I honestly can't imagine not using extensions. I'm 39 and have been on the web since Netscape etc in the early 90s and I honestly care more about the extensions than I do anything the browser actually does. Like, if there were no extensions I don't think I'd care at all if I used Firefox, Chrome, Opera, etc. But Chrome and Firefox have this massive, massive ecosystem of productitivy improving extensions. I'll give... - Source: Hacker News / 4 months ago

I think https://web.hypothes.is/ would be of interest to you. - Source: Hacker News / 5 months ago

Https://web.hypothes.is/ already exists for collaborative commentary on practically anything web based. So there is a market of sorts. Source: 11 months ago

If y'all are looking for something lower maintince, check out https://cirasync.com/. Source: almost 2 years ago

I use a third party app for this called Cirasync https://cirasync.com/. Source: almost 2 years ago