If you have been using Google Chrome, you should be familiar with its native PDF viewer. Google Chrome allows us to view PDF directly in the browser without no need to open external application. What about Mozilla Firefox? The Ubuntu's default web browser. Fortunately, you do it on Firefox too! Here's the trick.

Please install the latest Mozilla Firefox release on Ubuntu's repository (currenty version 15) and open it. Type "about:config" (without quotes) in the address bar, you will be prompted that it can be harmful.

Click "I'll be careful, I promise!" button. Now you should have a lot of configuration options on the screen. In the search bar, search for "pdfjs.disabled", double click on the search result until the value changed to "false".

Now, please restart your Firefox and you should have PDF viewer enabled on the browser.

Enjoy :)