While simply viewing or commenting on a PDF can be accomplished with free software, there are times ... it’s nice to be able to review and annotate files when you’re out and about.

So what I was looking for was a single app capable of pulling documents from Google, MobileMe and Dropbox, rendering them (even the large ones) with a crazy-fast engine and letting me annotate them.

There is a free PDF editor already available on ... move things around, and so on. The software also supports extensive annotation, including shape insertion, highlighting, underlining, striking ...