Not entirely sure but i think that you can do it by defining the .log extention as being of type application/octet-stream like this:
AddType application/octet-stream .log
within your apache2.conf or other config file.
This should ensure that the client browser doesnt recognise (and automatically display) the text content but instead prompts the user to download it instead.
1st post here - sorry if it is inaccurate or not what you were looking for