Adobe Reader Touch (I'm using version 3.1 on Win 8.1) holds files open even after apparently "closing" the app. This prevents Acrobat Pro, for example, from saving over the file, thus creating a bit of a nuisance. How does one force Adobe Reader Touch to let go of the file?
You may tap on the file and you may see navigation icons on the file itself, one at top and one at bottom.
Click on top one to go back application, that way you will close the file.