Marqueed is a service that allows users to create and discuss collections of images and PDFs. Using Marqueed you can highlight and draw on images as well as PDFs. You can share MarQueed files or keep your files private. Likewise you can create private and public collections of files.
If you upload a PDF to Google Drive, you can add comments to it. That comments won't be attached to specific parts of the PDF. The comments will appear in right-hand margin under the comments button.
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.