Hi,
It is possible to just store "links" for your product files but you will loose the HikaShop security.
Because the customer will be redirected to the URL you have specify, he will be able to copy that URL and re-use it (or share it).
If you want to keep some security, it would require a custom plugin in order to create a temporally URL, something that will just only work for few seconds (or few minutes). But it will require that the hosting platform have an API which allow you to generate such kind of URLs.
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.