Mac: Setting the default app to open a file on the Mac can be accomplished in a number of ways, including getting info on the file in the Finder and making your selection. If you want to get the job done quickly, just right-click on the file and choose "Open With" while holding the option key.
This will change the command to "Always Open With" so whatever you choose will be the default app for that particular file. When you make your selection, it will also open the file in that app so you don't waste a few extra clicks. For a demonstration of this shortcut, check out the video above.