Populus-Viewer is a tool for decentralized social annotation, built on pdfjs, wavesurfer.js and the Matrix protocol. You can use it to read PDFs, listen to audio, or watch videos, and have rich discussions in the margins, with your friends, classmates, or scholarly collaborators.

Each uploaded file is attached to a matrix space, and each annotation to the file becomes a room within that space. Populus-Viewer has been tested with synapse and dendrite, but should be compatible with any spec-compliant matrix server.

An online bookmark manager which also can act as a content archiver. Can save content as HTML, PDF, or PNG in addition to storing links for later reference. Automatic summarization and tagging of links. Multiple user accounts. Multiple back-end databases (SQLite default). Annotation support. Written in Python 3. Don't know if there's an API.