It would be helpful if you stated which version of Office you do have. In Office 2007 (for example), You can set the default format for saving files (so that you can guarantee that they can be opened by people who only have access to earlier versions of Office).
You do this as follows:
Click the stupid down arrow button near the stupid new File button at the top left.
From the menu, select More commands
On the side bar select Save
In the dropdown "Save files in this format" you will find a list of earlier formats in which you can elect to save a document.