the best format for archival purposes and for reading is
epub, the industry standard, and the 505 can read that natively. so if you can get that you should. it's usually sold as "adobe epub" if there is drm on it (this can be removed ; search for "adobe adept drm circumvented"). some websites (like BooksOnBoard) call the pdf format "adobe digital edition" so be careful not to get them mixed up, it's easy to do.
on BooksOnBoard, the
epub icon looks like this :
the
pdf icon looks like this :
if you can't get epub, the MS Reader (lit) format is a very good format to convert. you can also try mobipocket format or ereader, however the formatting and meta-data may not be as rich in these formats. for converting, you can take a look at calibre (see the dedicated forum for more info about that), it's a brilliant tool developped by one of our own members, for library management, format conversion, and rss feeds.