Word 2007 uses the new Word 2007 document format by default. If you work with people who use previous versions of Word, either you can save each document you send to them in an older format, or you can set Word to save documents
in the legacy format by default. Watch the video to see how to change the default file format that is used by Word 2007.
How to do it (text version): Save documents in a previous file format by default.