Hello,
You can still put an URL for your product file and the user will be redirected.
But that solution is not really great since the customer can know the final URL and re-download (or share) it like he wants.
Now, if you got a system which allow to generate unique and temporally download URL (I know it's possible with Amazon AWS) ; thanks to a custom plugin, you can see to handle it.
Personally, I made a plugin to handle "secure download" for Nginx and Lighttpd web servers ; which is based on a very similar idea.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.