Which program did you use to write the document?
I think that you probably wrote the document in Microsoft Word 2007, which has a different file format than the other words (97 – 03). Previous versions of word have the file extension “.doc” but 07 saves it as a “.docx” which the other versions of word cannot open.
If this is indeed your problem, you have two options.
A) Instead of saving it as a Microsoft Word 2007 Document, save it as a Word 97 – 2003 document. You do this, by, when saving, click circular button in the top left corner->save as-> word 97-2003 document
B) Tell everyone who you send documents to download the converter which is available on microsofts website
Hope this helps, if not, I’ll see what I can do