OS X comes with built-in PDF support. So this company’s ability to create and sell a Mac product for handling PDFs impresses me. Read their launch story here: https://medium.com/developers-writing/how-to-launch-a-mac-app-and-become-1-top-paid-app-globally-4434bbfb51ee#.5slv3m3aj
Reading between the lines, they seem to be selling $20K per day on the Mac App Store to US customers. They also sell directly from their own website.
I downloaded and tried their product. It is worth taking a look to see how a pro company does a great job of marketing software.