Hello,

Unfortunately we don't have full support for e-goods or downloadable products yet, we plan to include it in some of the future versions.

The simple solution would be to restrict the access to the document (file) and provide access credentials to the customer who bought the file.
You can use document permissions (described at http://devnet.kentico.com/docs/devguide/permissions_for_modules_and_documents.htm)

The permissions can be created and managed automatically using our API, however the concrete solution would depend on the scale (count) of the downloadable products -- e.g. you could have specific role for each such product (created automatically using Data or TreeNode Handler) and add users who bought the product to that one role and later remove them from the role after they download the file. Managing user roles using API is described at http://devnet.kentico.com/docs/devguide/managing_user_roles.htm.

Please note it requires at least professional edition to have ability to manage appropriate permissions.

Best Regards
Ondrej Vasil