Change add to cart to instant file download

Is there a way to remove the ‘add to checkout’ button and change it to an instant file download? I want free digital downloads within a members area but I cannot work out how to change it so the user doesn’t need to go through the checkout process. 

Please see the following.

I haven't tested the code with a direct download but if you can put the URL into the browser and the file downloads then I suspect it will work.

Let us know how it goes.