PDF is not very good for any device other than those with a larger screen. Taking a 8.5" x 11" page and shrinking it to a 6" screen will make the text very small. The Sony offers reflow but in reality only works for text based PDF.
I believe there are devices that will crop a PDF to get rid of the wide margins. There are other tools to remove the margins yourself before loading onto a Sony but I have yet to play with that.