First and foremost to encrypt documents you need some encryption program. For an asymmetric encryption you likely could use some PGP plugin for Mail, for example from GPG Tools. I think the attachment will be encrypted automatically by the plugin.
For a symmetric encryption you could create an encrypted .dmg (if the recipient is working on a Mac), or, for cross-platform, use the encryption option of 7z [1], or some other possibilities.
Once you know how you are going to encrypt the document, we can build a macro around it.
[1] 7z is available for macOS as p7zip, a very useful program. (Easily installable via Homebrew.)