I've heard good things about the PocketBooks and really like my Sony 650. Right now, the Pearl screens, used by the Kindle 3 and the Sony 350/650/950, are the best screens on the market.
PDF is a horrible format for readers, especially if they are images instead of text. The format is designed to render a 8.5x11 page exactly as it was created. Those images aren't going to work well on any of the 6" readers out there. Since these are free and out of copyright books, I'd recommend looking at the 18,000 books posted
here at Mobilread. You'll find that they are very well formatted and already in formats, like ePub and mobi, that do well on readers. People like HarryT take books from places like Google books and Project Gutenberg, clean them up, create tables of contents, rework the images, and format them to look good on ereaders.