If it's text based PDF, and it is at all complicated, or has things that I doubt calibre's ability to handle (e.g., mathematical notation and the like), then I use
SoPDF on it. This preserves the look exactly, but splits the page and removes the margins, which is usually enough to get a readable font size.
If it's a scanned/image-based PDF, I use PDFLRF to do more or less the same thing, which works pretty well unless the PDF had very wide lines of text.
PDFLRF and Rasterfarian are pretty similar, actually, but I find that I like my results a little better with PDFLRF.
But if you can get it into Word format decently, can't you just change the page and font size, and convert back to PDF? Use Sony's own recommendations here:
User’s Guide to Creating PDFs for the SonyŽ Reader