That is the (far fetched and pure fictional) idea: you send a portal. Like a door to the receiver. GMail f.e. can see the door, but not what goes on behind the door.

To see what goes on, receiver opens the door ( like a PWA URL?) Interacts through that portal, via encrypted channel... Afterwards closes the door... (Remove all temporary files and PWA history).

The PWA is where your magic happens, but for the receiver it is just like getting a mail, only that the back and forth is in the channel.

If you can have the service send a recap with the elements you want to keep as a follow up mail(non sensitive data, just as a reminder,... Meeting notes like)