A couple of more questions for you...
- What is your Android OS version?
- Have you been using Adobe Reader 11.1.3 (build 86198) for past several months? Or has it been updated to 11.1.3 on your device recently?
Adobe Reader for Android was released on January 14, 2014 (almost three months ago). It's not clear why the problem started to occur all of a sudden.
If you don't mind, would you completely uninstall Adobe Reader and reinstall it after restarting your device? As long as you have the PDF files on your internal & SD card storage, you will not lose the files when you uninstall Adobe Reader. But please backup your data files, if needed.
If the problem still persists, we may need to collect crash logs from your device. But let's see if reinstalling Adobe Reader can clean up some unknown Reader issues with your PDF files. Sorry for the inconvenience.