If you only want the body of the email, tap and hold anywhere in the email until you get the popup menu, and choose Select All, then Copy. Open whatever app you are using to create the document and paste it where you want it by taping and holding until the popup menu for paste appears.
If you want the whole thing, I only know one way (though there are probably others). An Evernote account comes with an email address you can forward the email too. It goes in the Evernote inbox. When I do this I get the subject, to, from, and other header information as well as the body in the resulting note. You can copy and paste this from Evernote to your other document.