Email content setting as per Doc

How to customize setting in Email for material request , Purchase order etc. doc type.
For e.g while creating Purchase order by User, After confirmation of PO the mail to be sent to supplier for further processes so while composing Email to Supplier the attachment of Pdf is automatically attached but If we want to add fix msg in main body

For Example:-
“Dear Supplier
We are happy to inform you that your quotation has been finalized by the management.
We, hereby, confirm our Purchase Order for the list of items attached with quantity. All our Terms and Conditions are mentioned in the Purchase order.
Hoping to have lot many further transactions with your organisation.”

How to add this in Setting , so that while creating Purchase order that particular content will automatically be there in Email

You can do that by using Notification, go to Notification doc.