PDF is not an eBook format, and should be avoided like the plague on portable devices. PDF is a digital representation of a printed page of a fixed size - generally A4 or Letter. It can only sensibly be viewed on a screen that's the same size as the page that it's formatted for.
For eBooks you should be aiming for a reflowable format, such as ePub or Mobipocket. Reflowable formats will adjust themselves to whatever size screen they're being viewed on.