Microsoft solves this with RMS for Individuals: https://portal.aadrm.com/ (also called the RMS Sharing App, now getting rebranded to Azure Information Protection).

The recipient's identity is the key to opening the content - no need to communicate anything out-of-band. Depending on the file format chosen, you get DRM features limiting granular actions on the file beyond view/edit.

To open the protected files, your recipients will have to download/install the (free) app from Microsoft. This is generally pretty painless.

Definitely worth checking out, especially good for consultants' workflows.