I love commenting on long-form documents. If the author put the necessary work into creating a cogent examination of a topic then I am going to put in the effort to understand it. And I do that by making comments. Some are questions. Some are observations. Some are petty, especially the typographic ones.
When you like commenting you want to have the space needed to comment. I seem to recall that I once forced an author to use double line spacing and an extra wide right margin so that I had room to write. That was great. When Microsoft Word added the comment bubble user-interface I did not think my commenting life could be better. I was wrong. Online collaborative commenting is even better.
Most of my documents are written using Google Docs. Docs has weak formatting and content structuring tools, but it does have great commenting (and version control). I am willing to lose a little structure and style for that. Not all documents come my way via Google Docs, unfortunately. Most of them use Adobe's PDF as authors want structure and style over commenting. Especially, it seems, those working in elementary and secondary education. Sigh.
Today I wanted to read and share my comments on RIDE's "Creating a Shared Understanding of Personalized Learning for Rhode Island," a PDF document. So I looked around the web for tools. There are a fair number, but I was happy to find that Dropbox allows for online collaborative commenting of PDFs. Just add the document to Dropbox and share it. Anyone viewing the document sees the comments and anyone wanting to add comments need only login with Dropbox or Google credentials.
Another online problem solved.