Right now their are two work arounds I’ve found to do this. Send a preview of the email to your own inbox, and then go to print email, but save as pdf instead. Unfortunately this way you will usually have all the email recipient/sender info which you will have to crop.
The other way is to download a chrome extension like “Go Full Page” and take a screenshot of the entire email, then crop, then save as a pdf.
Pretty sad that HubSpot won’t release this feature even though this thread is almost two years old with almost 100 replies asking for it. This is a basic feature that should have been provided out the gate.