If you want to learn how to open PowerPoint presentations without Microsoft Office this article explains different alternatives to the official Microsoft Office powerpoint that allows you to deal with PPT and pptx files and open powerpoint presentations on the way.
Microsoft powerpoint Viewer
This is a free tool that you can download from Microsoft website that allows you to open (but not edit) powerpoint files. This is the official viewer from Microsoft and is a free tool that you can download and install in any Windows based computer.
Inside SkyDrive or your Hotmail account you can access Office Web Apps and powerpoint online. This is a free tool that allows you to create and edit powerpoint presentations and other Office documents (Word, Excel) inside your browser. You can open powerpoint presentations by using web apps and just need to upload the ppt file to your Skydrive account.
In order to open powerpoint with online powerpoint you need to Add Files to your SkyDrive space. This will upload powerpoint files to the cloud (under your account) and then you can open the file in the web based powerpoint application.
Go to Office Web Apps
Alternatively you can use Google Presentations to upload and open powerpoint files, however using this approach your original powerpoint file will be modified at the time it is uploaded to Google. Although you can then download the PPT file this process will export a new copy of the presentation content and you are under the risk of losing some effects or transitions.
Open powerpoint in Android
In Android you can use Android applications to open powerpoint files, for example Office Suite Pro o QuickOffice HD Pro that enables you to open and see powerpoint presentations in your smartphone or Android tablet.
Open powerpoint in iPad
Also in the iPad you can open powerpoint presentations using different applications available in the Apple Store. Some are free and other are paid applications. Quick Office HD Pro is also available for iPad and iPhone.
There are many different applications and ways to open powerpoint, you can open powerpoint files online and offline, and also in your mobile devices, but sometimes it is good to learn about other powerpoint alternatives that you might discover.