Note: this is a "vintage" tip for older computers/systems.
Often you will want to tell Mac OS X which program to use to open a particular type of file. For example, you might want to use Preview instead of Acrobat to open PDF files. The following describes how to do it. Note that the procedure differs slightly for different versions of Mac OS X so this is just a guide.
Find a file of the type you want to associate with a program. Click on the file once. Choose Get Info from the File menu. Click on the "Open With" section (click the tab, triangle, etc). Choose the program you want to use from the pop-up list. Click the Change All button, and click Continue when you are asked to confirm the change.
Note that, because Mac OS X has multiple methods of identifying the file type, you may have to choose a number of files and repeat this before all are covered. Specifically the file suffix ".pdf" and internal signatures might mean the system identifies what seem to be the same type of file differently.