Comment by thangalin

Related: My FOSS tool allows uploading PDF files to a private server for annotating within a browser. Annotations are saved server-side in JSON format, which can be viewed and modified by anyone with the URL.

https://repo.autonoma.ca/repo/notanexus/blob/HEAD/README.md

The software uses PHP and PDF.js for displaying and annotating. Screenshot:

https://i.ibb.co/gL39qGdc/notanexus.png