This is most likely happening because you have too many files open at once. Please tap the icon at the end of the orange tab (looks like two documents on top of one another) at the top of the screen to reveal all the documents that you have open. Please try closing a few of your files so that they show up in multiple tabs across the top of the screen instead of one tab with a drop-down menu.
Then, please force-quit the app. To do this, double-tap the home button on your device and swipe away iAnnotate. Then, reopen and let me know if the problem persists.