Most documents posted on web pages are in some kind of compressed format.
Compressing a file allows it to transfer faster and also allows browsers to handle many different kinds of files without having to be told the details of each. The most common compression formats used on the web are the zip format, a Windows standard and not to be confused with Zip Drives, and the StuffIt format for the Macintosh. To uncompress the file, you need a program which will work with the browser automatically. Expander will uncompress files in just about any format on either platform. After the download is complete, you will find an installer on your desktop.