You can limit the amount of time your customers will be able to download their file by adjusting the value of the File to Download product attribute, included in the Digital Downloads package.
To do this, go to Dashboard > eCommerce > Settings > Attribute Settings and click the Product Attributes link. From the list of attributes, click File to Download. The Edit Attribute screen will appear. Check the the checkbox to enable Download Time Constraints and enter a value in the Days Available field.
After a successful purchase, the customer is directed to a landing page where they'll be presented with a link to download the file they've purchased.
Customers will also be sent an email with a link to download their file:
If a visitor does not have the proper concrete5 cookie from the checkout session that made the purchase-- they'll see this message instead: