Try Crocodoc for Collaboratively Annotating PDFs

Yesterday, I received an email from a reader who was looking for a free tool that she and her students could use to collaboratively annotate PDFs. While you can comment on PDFs in Google Drive, you can't yet anchor those comments to a specific part of a PDF (at least I haven't figured out how to do that yet). The tool that I recommended instead is Crocodoc.

Crocodoc is a simple service that allows users to quickly share and edit PDFs, Word documents, and PowerPoint files.To use Crocodoc just upload your file, select your marking tool, and get to work. Crocodoc provides a unique url for every file you upload. Share that url with the people you want to have comment on your PDF, Word file, or PowerPoint slides. You can also embed your file into a blog post or webpage and allow people to comment on it there.

Applications for Education
Crocodoc could be very useful for online peer editing of documents. You could also embed a document into your class blog and ask students to comment on what they're reading. You might also use an embedded document with annotations to model proper peer editing practices.