If you use Epub, you can use custom fonts by pasting some CSS code in Calibre for the PRS-300:
https://www.mobileread.com/forums/sho...&postcount=1a8
I'm liking it so far. I've replaced the default fonts with something a bit bolder/thicker, so it doesn't look like it's half-erased in sunlight and is easily readable while in a moving vehicle.