Link: Presentation.io | Sync presentations to all devices

Cost: Free [However there is a paid-for service offering more functionality]

Live broadcast your presentations to mobile devices, tablets and laptops of your audience - no matter whether you are in the same room or in different parts of the world...
...Add comments to the slides as you go. These will also appear on everyone's screens real-time. Enable the audience to comment as well. For additional remarks or information, both the presenter and the audience will be able to make individual and shared notes during and after the presentation...
Upgraded [paid for service] includes the option to keep your presentations on the website for longer than 48 hours [the limit with the free version]

Supported file formats: PowerPoint, PDF