PDF Xchange Viewer is an awesome little product that not only lets one view a .PDF file, but more incredibly add comments and annotations on top of any PDF file. It's now easy to open any .PDF file and add: Text Text boxes Shapes and figures (arrows, lines, etc.) Colors Just about anything to emphasize and add clarity to a previously Read-Only file. Unfortunately it is Windows only, and is not web-based. However, this tool is well worth the effort to download and  install. Try it out.

Following is the Default Toolbar for PDF-Xchange Viewer. Notice the variety of markup options.



The following illustration is a .PDF file that was opened in PDF-Xchange Viewer --- and 30 seconds later it was fully annotated with a Draft stamp, colored arrows and boxes, etc.



For more details (and other exciting products), visit their website.