Pays attention to the context of a presentation's usage
— For a presentation sent via email, PDF format suits best (with file size below 10 Mb).
— For live speeches, PPTX format is preferable, as you can insert animations and videos on slides.
If a deck will be shown on a plasma screen, laptop or modern projector, the slide size should be widescreen (16:9). For old projectors and iPads — 4:3.