What makes it stand out from the other PDF annotators in this list is that it allows you to share your marked-up PDF file with a shortened link, and even with embed codes! Once you share it, you can choose to let your viewers add comments to your document. The other unique feature is that you can collaborate with others so you can all view and annotate the file together, even if you're in remote locations. You can then see all of the changes made to the document in a sidebar. Crocodoc allows you to download the original file as well as the annotated version, which you can view in Sumatra, Evince or your favorite PDF reader, with annotations and all.
This little app made my day! If you have ever been frustrated by having to type in a long url or wanted others to type one in in a hand held device, without email etc.. this is the app for you. Copy and paste in the url. ShortEnuf will generate a 5 character url that lasts all of 3 minutes and allows everyone to get the page you want them on painlessly!