Despite what @ManoaHI wrote, the app works just fine in any iOS environment, without having to get a different app. It's universal.
I've been using it for several years as well, and it has grown from a simple read-only PDF manager to the full-fledged tool that it is today. I'm sure there are other plans on the horizon, so I'd grab it, not because Apple may crush it (Why would they, @ManoaHI? Apps like this makes the iOS platform much stronger for doing actual work!), but because the price goes up as more and more features are added. They have to cover their added investment in feature implementation, but as an owner, we don't have to pay more for future upgrades!