Adding Line Break to Email Body Content of Mail.app Message Generated in AppleScript
If you interactively type the message, then your shell should allow you to continue a string on a newline, as long as you don't close the quote.

For example, type this and the outgoing message will have three lines.

osascript /Users/dwm8/Desktop/email.scpt "Test" "dwm8:test.pdf" "Test
&gt; Message
&gt; Here"


You can do the same thing in a .sh script, 

#!/bin/sh
osascript /Users/dwm8/Desktop/email.scpt "Test" "dwm8:test.pdf" "Test
Message
Here"