In order to view the file again, you need to uncompress it. Here's the info you need for the compression formats used on this site. The .zip ending on files means that the file is compressed in the ZIP format. ZIP is a flexible format allowing multiple files, and even whole directories to be stored in a single, highly-compressed file. Versions of zip are available on virtually every platform, but you may have to look around.
Programs are available for other platforms as well.