This is actually non-issue, but for the technical ignorance of most of the legal community, which *still* prefer protect their emails with a disclaimer in the signature rather than using public-key encryption.
There are two levels of protection: reasonable and technical.
If you are simply trying to satisfy a court's definition of "reasonable" then zip up your documents with a password before sharing them.
If you are striving for technical protection, that is to say if you have data that must be shared whilst being protected from prying eyes, whether inadvertently or maliciously, encrypt your documents with the highest-quality military-grade encryption allowed in your jurisdiction. Then you shift the burden of protecting your secrets from an outsider to yourself, and you know the job has been done correctly.