Because as of Word 2007, a new format is used to store your document
in. This means that people with older versions of Word have to
download the compability pack (
http://www.microsoft.com/downloads/
details.aspx?FamilyId=941B3470-3AE9-4AEE-8F43-
C6BB74CD1466&displaylang=en) to be able to see your files or you have
to store them in the older format (Office button -> Save As -> Word 97
- 2003 Document).
Note that if you use the older format to store your files in, some of
the new features such as equations and citations are not available.
Yves