Product file not showing

When you upload a file via the "files" interface, that file is only available after the purchase of the product on the order page. That is used to sell mp3 or ebooks for example.
If you just want to add a file to your products so that it's displayed on the product page, you can either put your link directly in the description of the product, or create a custom field of the type "file" for the table "product" in the Business edition in order to have that possibility added to the product edition screen.