PDFs display on a 505, so I'm not sure why you necessary want to change formats. Sometimes it's enough just to strip the whitespace with something like SoPDF.
Try the following tools with various kinds of PDFs and see what you like:
PDFLRF
Rasterfarian
PDFread
SoPDF
PaperCrop
(Some of these convert to LRF; some leave as PDF but make the PDF more readable.)
Personally, I can get most PDFs that start from already published material to display just fine, though some of these, by converting to images, take up a lot of file space.