There are several screen readers available, these are high end products used by totally blind and visually impaired user to operator a computer. JAWS is preferred by all the blind computer users I know. According to this there is a version designed specifically for tablets.
Be aware, the creators of PDF documents, often do so without consideration for ...
Check Adobe Reader's internal volume:
Go to the Edit menu, then Preferences....
On the left, 2/3 of the way down, is the category Reading. Click on that.
You should see a box labeled Read Out Loud Options.
There is a Volume selector ranging from 1 to 10; make sure that is high enough.
I recomend FBReader for opening the files with FBReader PDF and TTS+ Plugin. There are other applications you can use especially if you are willing to pay.