Quick Look enables apps like Finder and Mail to display thumbnail images and full-size previews of Keynote, Numbers, Pages, and PDF documents, as well as images and other types of files. These previews can be tremendously helpful when a user is trying to locate a specific document.
Implement a Quick Look viewer in your app if it makes sense. If your app lets people attach or otherwise interact with files—especially files your app doesn’t natively support—consider implementing a Quick Look viewer so people can preview those files without leaving your app.
Consider implementing a Quick Look generator if your app produces custom file types. A Quick Look generator enables other apps, including Finder and Spotlight, to display previews of your documents, making it easier for people to find your content.