The configuration of the visitor's browser determines what happens when a link to a file is clicked. You have no control over what happens and can't "force" anything. If you want your visitors to download a file as a file rather than opening it in the browser window tell them to right click and "save as..."
You have to add these lines to your .htaccess: <FilesMatch "\.(doc)$" > ForceType application/octet-stream </FilesMatch>