Don't know of any tutorials. But I suppose what you need to do is send a multi-part mail with an RTF encoded part. RTF is a human-readable (and writable) file format and there's a third party PHP class for creating documents.
Details on sending muti-part mails can be found here.
The RTF MIME type is: "application/rtf".
That PHP class is here.
And other details of writing RTF documents can be found at microsoft.com and oreilly.com.
