You can save a file in a different format so that people can read it even if they don't have the application you used to create it.
Some formats also make it easier and quicker to send a file via e-mail.
You can also change the format of a picture so you can use it in different applications. These instructions should work for different applications and operating systems. Open the file you want to change by double-clicking on it. Click on File. Click on Save As.