Make sure a ".doc" extension is added to the end of the filename of the Word
document.
Some other compatibility issues are mentioned here:
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http://makeashorterlink.com/?M40A25866>
This process might work to open the files: Tell the user to open Word on
the Windows machine, and then go to File -> Open. Tell him/her to choose to
show "All files (*.*)", and then tell him/her to navigate to and open your
Word document.
The problem could be caused by the way you are sending the files. Are you
sending them by e-mail? Is compression turned off? Are you sending in
encoding that does not include a resource fork?