Most documents have a default application that will launch if you open that document. You can also specify an application if you wish to open a document with something else. You can do this with drag and drop, or by using the context menu. You can permanently change which application is used to open a specific document. You can also change which app is used to open all of the documents of the same file type. If you want to find other apps that can work with a document, it is easy to search for them in the Mac App Store.