When you try downloading one of the Microsoft Office file types such as .docx, .pptx, .xlsx, the file may download as a .zip (Compressed Folder) file. This happens if the Office document MIME types are not registered in the web server you’re downloading the file from.
There is nothing much the user can do, apart from saving or renaming the file to the correct extension before opening it using Office.
If you had already downloaded the file (as .zip), simply rename (change) the extension accordingly. To do that, you’ll have to clear the Hide extensions for known file types check box in Folder Options.
If you’re a Webmaster and want to register Office Open XML File Formats on your server, check out the Office file format MIME types.