A few steps you can do in order to make pdf files more readable on your iRiver Story HD ebook reader.

1. Remove margins. White margins take up unnecessary space which restricts the maximum possible font size.
Use pdfcrop to do this in Ubuntu. sudo apt-get install texlive-extra-utils
(from http://alexsleat.co.uk/2011/01/25/using-pdfcrop-to-remove-white-margins-ubuntu/)

2. More to come…