Hi,
Since version 1.4.4 of HikaShop, you can order free products if the option "Display 'add to cart' button for free products" is turned on in the configuration.
However, the status of the order won't allow the user to download the files attached to the products in the order since the order status will be created. You can of course allow downloads for that status but then, users could also download the files of the paid products without paying by just creating the order.
Being able to download the files of the free products in non confirmed orders will require a small modification. You will have to edit the file show of the view order of the front end via the menu Display->Views and change the line:
if($this->order_status_download_ok){
to:
if($this->order_status_download_ok || bccomp($product->order_product_price,0,5)==0){