One of the most frequent questions we get is how to open files with a particular app. This most commonly occurs when someone has discovered that Preview is so much better that Adobe Acrobat reader but whenever they click on a pdf file it opens in Acrobat. Another common one is when folks that have been using Microsoft Office for a long time but find that Pages and Numbers serves their needs just fine.
Well, it is easy to tell your Mac which app to open what type of files. If you want to open a file in an app other than the default, select the file and press Command + I or choose Get Info from the File menu. Scroll down to the Open with section and use that drop-down menu to choose the new app that you want to use. In the case of making sure that your Mac opens pdf files with Preview instead of Acrobat, simply select Preview to open that file and click on the box below that says: Use this application to open all documents like this one. Mac will ask you if you are sure and once you confirm you are done and all your pdf files will be opened by Preview. The steps are the same to open MS Word docs with Pages or MS Excel docs with Numbers.
To reverse the process you just have to select the original default app in the Get Info screen.