Short answer
HubSpot's KB: export PDF or HTML versions of marketing emails. In the editor, use File > Export email design; on a sent email's performance page, use Export > Export email design. For A/B tested emails, pick which version. HTML helps recreate the email elsewhere; PDF suits approvals.
1. The File menu
The email editor in my demo portal.

2. Related
3. For approval
See content approvals.