This project only covered two methods of scaling to a tablet interface. There are, of course, myriad ways of improving what we've shown here, or using other design patterns to scale to a tablet interface. Why don't you try a few?
The split-view pattern has the left split view (or master view) disappearing when in portrait mode. When the Documents button is tapped, it appears immediately. Why don't you add some animation instead, to make this less jarring? (Don't forget to animate it when Close is tapped too.)
Continuing the theme, dismiss the left split view automatically when you select (or create) a document when in portrait mode.
Most apps today will allow a gesture to open and close the sidebar (usually a horizontal swipe). Add this to the app.
Lastly, when the app opens in portrait mode, there's no real indication of what to do (short of tapping Documents)—make the sidebar appear automatically.
Instead of using Create buttons to create a document...