Ever wondered why you have to open up a PDF in Android on Chrome or using a 3rd-party viewer? Well, it's because up until now Android hasn't had a native PDF rendering tool in place. As of Android "L," it does. If you're on Android 4.4 or below, try opening a PDF in the Drive app - you'll be sent to whatever your native PDF viewing tool happens to be.
Now, if you're on the "L" preview release, do the same thing in Drive (make sure no PDF viewers are installed, that might break this behavior), and you'll see the PDF displays natively in the app.