When the iPad came out, GoodReader sold like hotcakes. It was one of the very few apps that allowed iPad users to read .PDF documents on their new device, and it was priced at $0.99. It still is, actually. GoodReader was full of features, maybe too much for an app that didn’t really care about having a “plesant” UI and user experience. For weeks, people were forced to use GoodReader - there was no better alternative.
Eventually Apple shipped iBooks with PDF support, and a couple of new interesting 3rd party readers were released in the market. Fast PDF, for example, is the app I’ve been using all along to transfer documents on my iPad (and iPhone) and read them.
Folio, universal for iPhone and iPad, aims at becoming your default choice for importing documents, read them and organize them.