In most cases, double-clicking a file on your Mac automatically opens it in the appropriate application. But sometimes you may want to overrule your Mac and open a file in something other than the default.
For example, say you’ve edited a series of images in Photoshop, and now you want to take a quick look at them. You might prefer to view them in Preview, a Mac OS X program that opens in an instant, rather than the larger, slower-to-load Photoshop application.
To quickly specify your app, Control-click the item you want to open, then choose Open With from the pop-up menu that appears. This takes you to a list of every application your Mac considers capable of reading the file. Choose the name of the application you want, and the file opens in that program.
If you think you’ll be opening the file repeatedly in that program, you may want to specify an ongoing Open With preference. To do so, select the file and press Command-I to see the file’s Info window. Click the Open With tab and choose your program. Now the file will always open with your preferred application. And if you click the Change All… button, every file of the same type will open with this application.