As a 505 owner I must recommend against buying either Sony product if you want to read large amounts of PDFs (of supposed varying complexity). The 500 will view pdfs but unless specifically formatted for the screen (you could do this by printing the documents to a software PDF-printer) it will be nigh-unreadable. The 505 with the latest firmware updates offers a reflow hack which allows proper font resizing but this leads to weird sentence and paragraph breaks - IMHO it is a little unreadable. But better than the 500.
As for converting, there are various tools out there but depending on the complexity of your PDFs the end results may be less than spectacular. You may want to look into a reader with a larger screen, or investing in a different format
(disclosure: I loathe PDF as a format).