I actually bought PDF Expert on the Mac App Store via this sale, despite already having a “direct sale” license that I had bought awhile ago through StackSocial or one of those other deal sites awhile ago.
The reason is that I recently went to install PDF Expert on a new Mac and it told me my license was invalid.
Long story short, it’s only good for 3 installs, and if you don’t de-activate it before wiping/reinstalling, you’ll have to contact their support team to get it reset for you. It took multiple days and multiple emails to get this straightened out.
One of the advantages of the Mac App Store is not having to deal with that sort of nonsense.
I’m not sure PDF Expert is any better then PDFpen, which I get through Setapp, but I like having them both around.