Replace the add-to-cart button in Easy Digital Downloads with a direct link to the download file when the product is free and has only one file. This only works when the file is an external link to a web-accessible file.
For archive pages on the Shop Front theme, the cart icon is replaced with a download icon and direct download link.
Do you want to see what it looks like? Take a look around this shop, it’s running this plugin! Wherever you see a free product, you’ll see a simple Download button instead of the Add to Cart button.