It would be really great to have a thing for commenting on PDFs.
PDFs are a defacto paper of the internet. They are so hard to edit that they seem permanent, if you are reading papers they are generally PDFs. If you print out a paper, you can write notes in the margins and highlight things, and then show those to others.

But is there an easy to share commenting-on-documents system on the web (or better, the decentralized web...)

I like how google docs does it, but googdocs is also really slow and heavy for my slow computer.

you might be interested in Peer Library, made by @mitar and friends.