The pdf contents appear in what you see so you can "preview" it on your screen before sending the message. The pdf file is actually sent as a file attachment when the mail is sent, which I think is what you what.
That said, when composing your email, if the pdf file contents appear inside the body of your email and you don't want that, you can right click (or option-click) on the pdf file that's in the body of your email and select "View as Icon" and then it will just show the file without displaying the contents all over your email.
By the way, this works with received emails as well.