Pick a PDF file you want to read:
On the top right of the listening screen, tap the 'reading mode' icon to show the file's original view
Tap 'Aa'. This should open up several options for you.
The third and 4th row, lets you change the font and size that you want to use for your reading pane.
Tapping the font name gives you 8 different options which you can choose from.
For the Font size, tap the '+' to make text appear larger, or '-' sign to make the text smaller
