With Apple’s Mac OS X Leopard, you can open many files in programs other than the ones in which they were created. For example, if you’d prefer to view an Adobe Acrobat PDF in the faster-loading Preview application, simply select the PDF file, choose Open With from the File menu in Finder, and choose the Preview application in the pull-down menu.

But what if you always want to open PDFs in Preview instead of choosing this application each time? Mac OS X Leopard offers an easy way to reassign all documents of the same type to open in the application you specify.

Let’s say you’ve saved several Adobe Photoshop files as JPEGs. In the future, you would like to open these files using Preview, rather than waiting for Photoshop to load. To reassign the default application simply select any one of these files, then go to the File menu in Finder and choose Get Info (or just select the file and type Command-I). Click the Open With disclosure triangle, if needed, to reveal a pull-down list of all the applications on your Mac that you can use to open this type of file.

Choose Preview from the list of applications, then click the Change All button beneath the application pull-down. (This button is grayed out until you select a different program than the one already assigned to open the file.) A pop-up will ask you to confirm that you want to apply this change to all documents of this type. Click Continue. Now whenever you double-click on one of these JPEGs, it will automatically open in Preview on your Mac instead.