Download Now for variable products

I’m looking at this plugin for use with an online shop which sells paperback books and PDF ebooks of the same (some free, some paid).

If I create a simple free product for the PDF book it works like a charm and there’s a single Download Now button. No collection of personal info required.

However, if I create a variable product with variations of the paperback (at cost) and PDF (free, virtual and downloadable), go to that product page and select the PDF I see the Add to Cart button. Going to checkout from the cart requires the user to fill in a form with all payment details, even though it is free.

Is there any way to get the Download button appearing for variable products where the free download is chosen?