SnagFilms is a free streaming documentary app. Not only is the app free but you can watch the documentaries for free as well. There is a short ad that you have to watch at the beginning or every time you stop and start the app but it's only about 30 seconds.
I have browsed and found several documentaries that were of interest to me including the first one I watched, Kurt and Courtney, on the late Kurt Cobain. Once you are watching a show it puts in the section My Films, which is nice because some times they can be hard to find again. Most of the time once you exit a show, it will start up where you left off however sometimes it doesn't so it's best to remember how far along you were.
Overall this app was free and has over 80 documentaries. My only grips are that it sometimes loses your place when you start it back up and there is no search feature. However the app works great and is worth taking up space on your iPad.