PDF Reader with MP3 player? I'm learning a new language using a PDF book and audio dialogues on an Android tablet. My problem is to have a MP3 player near my PDF viewer so I can start/stop audio reading text. 

Is there an application for this? Or, is there a method that simplify respect quitting PDF reader to pass to MP3 then reopen PDF reader?
As long as your tablet is running Android 4.1 or newer then there are a number of music players that can put player controls in the Notification area on your device. This is so that you can just access the notification bar while you're running another app, without exiting the original app.

I haven't got a screenshot handy of how it looks on a tablet, but here's an example of one on a phone:



Try searching the Play Store for something like MP3 Notification Bar Controls