When you double-click on a file, it will open in the program that the computer's operating system deems is the most appropriate. You can set the default program for a specific file or all files of a specific type. Changing the default programs for a file type in Mac OS X is intuitive and easy. Changing file type associations is done in the same way for any file type on your Apple operating system.
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Open a Finder window and then select the file that has the default program association you wish to change. Click the "File" menu and select "Get Info." Alternatively, simply press "Command" and "I" on the keyboard.
Click the "Open With" drop-down menu option, which currently displays the name of the file's default application.
Scroll through the applications in the drop-down menu and select the application you want to open the file with. Changing the application changes it only for the file you have selected.
Click the "Change All" button below the Open With menu if you want to use the new application to open all files of this type. For example, if the current file is a PDF and you selected "Preview" as the new default application, this will make all PDF files open with Preview. A dialog box opens, warning you that you are changing the associated application. Click "Continue."