Hi,
If you add a file to a product in its "files" area, after an order is confirmed, the system will send an order status notification email to the customer and a download link will be included below the product name in the email.
Also, a similar link will be available on the orders history of the customer on the frontend. You can create a menu item of the type "order listing" or "user control panel" to allow the customers to access that area.
Also, you can also activate the "Enable customer download area" setting of the HikaShop configuration to be able to access a download area from the user control panel (or create a menu item of the type download area once you activate that option).
And finally, you can also activate the "Purchased file download on product page" setting of the HikaShop configuration so that the downloads will directly appear on the product page of the product on the frontend once the product has been purchased.