It's really rough though.

https://github.com/burtonator/pdf-annotation-exporter

Basically I have it working with:

https://mozilla.github.io/pdf.js/web/viewer.html

Whereby I can paste the pos.js into the console and it can export PDF annotations with screenshots of the annotation.

I intend on using this with Anki to read more complex books and take notes with it.

Now I just need to wire it all together with chrome headless and PDF.js