By default, all files on a Macintosh computer are assigned an application that will open them if you simply click on the file. For example, double-clicking on the file meetingnotes.doc may open the document in Microsoft Word, even though you also have Pages or Open Office that are capable of reading and editing the file.
You can permanently change the default application that opens any type of file. Learn more on Apple Support - Choose an app to open a file on Mac.