In OS X, all file types have a default application that opens when you double click on them. If you double click on a PDF file or a PNG file, chances are that your Mac will open it in Preview, Apple’s default PDF and image file app. If you’ve given an app like Adobe Reader, for example, permission to set itself as the default PDF app, then all PDFs will open in Reader.
Over time, you may have set apps as default that you no longer want to open your files. Conversely, you might want all JPG files to open in Preview, except one specific JPG file, which you’d like to open in Photoshop. Here’s how to make both of these situations work for you.