If you haven't checked out Calibre for conversions then you should.
As others have said - the issue with PDF's is that they assume A4/Letter size. When you're reading on a smaller screen (especially a basic e-ink one) the content is much more important.
But a PDF doesn't have to be A4/Letter. I published a book recently (ebook in PDF, ePub and mobi and paperback) and decided to use 6.69 x 9.61 inches for the PDF. I found that this size looks good on paper and on screen.
