Encrypting Office and other files with a certificate

Hey guys,

I'm trying to find a way to encrypt files so they can only be opened by users who have the assigned cert.

We don't want to just do password protection on the files, or use EFS, as they're not strong enough encryption.

I'm talking about encrypting pretty confidential documents, so we need something hefty.

Anyone have any awesome suggestions?

Thanks,

Rhys