You can give site users a template that they can download on the product page. You would use Assets to do this (Sites > [site name] > Assets). You would set up the Asset as follows:
Usage for = Product
Product = Product which needs to have the downloadable file
Customer can download = True
Location of asset = Upload Asset (Then upload the file in the Upload Asset field) or URL to Asset (Then set the location of the asset in the External URL field)
This will add the downloadable file to the product page.