You can write the content of the output template body in various formats including:
For improved security in your org:
You must select the appropriate content type for the output template body in the output template. See Output Template Fields.
If you are creating a PDF template, write the content of the output template body in HTML. If your content uses images, they must be in the documents folder in your Salesforce organization and the image tag must point to that location. You cannot reference content from an external website.
Reference
Output Template Body HTML Definitions
Output Template Fields
Sample Microsoft Word XML Body
Sample Microsoft Excel XML Body
Sample HTML Body