PDF Reader with MP3 player?
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