Hi,
You need to add your files in the "files" area of your products. That way, your users will be able to download them on the order details page after their order is confirmed.
Normally, in the confirmation email they receive, there is a link to that order details page (unless you set hikashop to have "no registration"). You can modify the text of the emails to make it more explicit that the file can be downloaded on the product page by changing the text via the interface in the config under the tab languages.
To display the order details page at the end of the checkout, you can do like explained there:
www.hikashop.com/support/forum/4-how-to/...t.html?lang=en#19625