Either the person opening it on the other end doesn't have the same program, or you're saving it on a system that the other computer doesn't support.
For example Windows Vista saves documents on a default of .docx files. These won't open on older versions of Windows, such as XP. When saving the file, select File > Save As... and save the documents as Works 97-2003 document (or something like that).
I had a lot of problems with that at first, so hopefully I've helped in some way.