Putting open things under the same app on the Dock

Hi, I have a MacBook Air and I know some of my friends with MacBook Pro can do this:

When I have multiple PowerPoints, or Word Documents, or PDFs open they all have their "own spot" in the Dock, but Pro users have like all the powerpoints in one icon and when you click on it the different open powerpoints are displayed and you can choose which one you want to open. How do I get that to happen on my Air?

Hope all that makes sense. Thanks in advance