Comment on Skim

A FANTASTIC app. I've used it to mark up research papers and it's great.

One complaint I see on this page is that you can't view annotations you make with Skim in other PDF editors. That's because Skim doesn't modify the actual PDF. It stores the "skim notes" in a separate file. You can still export the file as a PDF with embedded notes though... so you CAN share the annotations with others.