Beyond the print functionality that's already built into web clients like Outlook, I don't know of a way you can really do this with Marketo.
One way (fair warning, this may be a bit of a pain) could be that you create a PDF of the email beforehand, upload it to your Marketo instance, and then link to that in the header of the email. It would be time consuming because you'd have to do it for each email you send out, but since Marketo doesn't have functionality that will auto-generate a PDF for you, that's probably the closest you can get if a PDF link is what you need.
Alternatively, you could simply change the text for the "view as web page" link (located in the Admin settings under "Email") to something like "View as Web Page and Print". Once they open it in their browser, they can simply use their browser's print functionality to manage the rest.
Hope this helps!