Hello Kevin,
even though you are not opening the file via an iview, it's probably still the same solution in 7.4 as it was in 7.3 and 7.31 (at least in our portal I need to maintain that after pretty much every SP update and after the different upgrades. ^^):
You need to change the authentication scheme of the KM Document iView-template. If I remember correctly, you should set it to "Basic authentication".
Regards,
Steffi.