Alliance Partition Systems
Presentation Orientation Review

(Recommended) The horizontal PowerPoint format is the industry standard for presentations on laptops, computers, and clients' screens. These are used as pitch decks.
Click for a horizontal test.

The old standard was vertical. It won't work well for digital presentations. 
Click for a vertical test.

*Amended 5/2:
The letter size orientation in PowerPoint does not work well for digital presentations as you need to scroll down page by page rather than across as a slide show. Here is an example of a vertical letter size orientation that would need further work for vertical printing to fill voids.