Control-clicking a document to see the Open With contextual menu is a handy way to choose exactly which app you use to open a PDF, JPG, or any other file. OS X has a problem, however, where duplicate apps can show up and make the list grow longer and longer. You can fix that issue when it crops up as long as you don't mind a quick trip to Terminal.
OS X duplicates apps in the Open With list over time
To reset the Open With contextual menu app list, launch Terminal (it's hiding in the Utilities folder inside the Applications folder) and enter this command:
/System/Library/Frameworks/CoreServices.framework//Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -domain local -domain user
A quick trip to the Terminal app can fix the duplicate app problem
Now press Return. The command forces the list to reset itself and drop the duplicate apps so you see only one of each. Sadly, this isn't a permanent fix, so don't be surprised when you have to use the command again in a few days or weeks.