Although it is a bit large (it has a 3.5" x 4.75" screen), the Sony E-Book reader will display PDFs. In addition to PDFs, it will allow you to view text files, RTFs, and pictures. It also functions as a basic MP3 player (just plays the tracks in alphanumeric order) and will play MP3 and AAC files. Although it only has a modest amount of flash memory on board (I think it's about 64MB), more memory can be added via memory cards.
One of the best things about this e-book reader is battery life. In regular use as an e-book reader you can go a few days on a charge (I went about 2 weeks on my first charge), although my experience has been that playing audio files will reduce the battery life.
I hope that helps.