I presume these eBooks are set up as two separate products in Seller Admin? Make sure you set the free eBook price to 0.00 -- if a buyer adds only that free product to their cart and nothing else, that should offer them a Free Checkout to obtain their download link.
If a buyer also adds any other, regular-price product(s) to their cart along with the free eBook, then the buyer would need to checkout with PayPal to pay for the non-free product(s), and then they will get their download link for the free eBook along with their non-free items.