Hi,
After your customer complete the payment process, a confirmation email will be sent to your customer containing the download link so that he can directly download the file with it.
Also, I think that the best solution to handle download file will be to enable your customer download space, through the "Enable customer download area" option of your "Hikashop->System->Configuration->'Main'" page.
The download space will allow your customer to see all their available download files and download them until they reach the download limit.
This download space will be available through your customer "User control panel"