Zamzar is a free online file conversion service that is very easy to use. You upload your file to their server, select what file format you want it converted to, enter your email, and done. They will send you an email with the link to the converted file.
This is a very useful site in these days of different operating systems and software versions. Students use different software than school and then the files can't be read on the school computers, etc.
The free version can take a little bit of time to convert, but it's free. There are premium fee versions that allow you to have faster conversions and create accounts to manage your files.
Zamzar can convert literally hundreds of file types, including Microsoft Office, PDF, ODP, and much more. It can even convert Apple Keynote files to MS PowerPoint. The conversions include documents, images, music, video, e-book formats, compressed files (like zip), and even some CAD files.
Here's a listing of all the file types.
This is definitely a tool that everyone needs to have.