Hands down, Goodreader is my absolute "Must Have" app. Goodreader is like my mobile file cabinet. For every document that I store in Goodreader, I create a file for that particular transaction, buyer, seller, etc. That way I don't have to search through an exhaustive list to find the document that I am looking for.
In addition, I can markup the document by highlighting, adding text notes and much more. Sure there were other mobile apps installed on my iPad and iPhone before I even discovered Goodreader, BUT I would use them sporadically. With Goodreader, I use it methodically. I is by far, the most used app on my mobile devices.
When I am done with a file or a transaction has closed, I'll keep the Goodreader file for 1 to 3 months, then make sure that I have printed everything, then I'll remove the file from Goodreader to free up space on my device.