MS Office has tools specifically designed to delete metadata. http://washburnlaw.edu/library/technology/tutorials/metadata-win.html is but one of many places, including official MS sites, which shows the procedure. It's been around in one form or another since at least Office 2003.
Using LibreOffice or similar would also clense the metadata. Hell, WordPad might do the trick, I've never tried it but it should clean the Word metadata.
And there's always the good old use a different computer.
If _I_ were running this scheme, I'd write the invoices up using LibreOffice in a encrypted VM. Do the work in ODT format, export to DOCX, or, better, to PDF. Store the VM's VHD on a external drive. Run the VM only on someone else's machine.
Not stealing $6 million would also work, but obviously wasn't an option.