When you have a large number of documents open iAnnotate will consolidate your tabs into a drop down menu.
Tap the grey pages icon on the right side of the orange tab to open the drop down menu.
To return to tab view, simply close a few of your documents and reset the view either by returning to your main library or rotating your device from portrait to landscape (or vice versa).
If you see a triangle with an exclamation point in place of the pages on the right side of the tab this mean that you have too many files open and the app's performance could be affected. In this state it is recommended that you close as many files as you can by tapping the "x" on the right side of the document name, only keeping the files open that you are actively working on.