Keynote works, but it is expensive, and expect loss of formatting when converting from powerpoint (typically in the form of shifted boxes).
The earlier poster's suggestion of opening directly from dropbox can work, if you don't mind missing out on all the animation and just want a no-frills document version. 🙂
Alternatively, save the ppt as pdf, and copy it to ibooks or some other pdf reader app like goodreader. A quicktime movie file can work as well.