Google Book Search is now supporting downloads of over 1 million public domain books through the EPUB format. EPUB is a free, open-source standard supported by some e-readers.
EPUB is text-based and allows text to automatically conform to smaller screens. Many e-readers don't render PDFs too well. Google has taken a lot of time digitizing books from print to the EPUB format.
To use the format, first conduct a search for a public domain book within Google Book Search. Then look for the "Download" link in the top right corner. Click on it and a menu will pop up. Select "EPUB"