If you want to offer access to protected files and pages upon an order, here's a popular way to go about it:
1) Upload your downloadable files into a Private Folder. This will restrict access to them from any User that does not have access to the Folder.
2) Create a Page with any post-sales information and a link to the downloadable file and put the Page into the same Private Folder.
3) Create a Product and set the "Grant Private Access" to the same Private Folder.
4) Add a message and link to the Private Page in the Product's Order Receipt Message area (which will be displayed on the Order Receipt page).
5) Place the Product into a Product Group, assign the Product Group to a Catalog or Order Form Page (however you want to display it).
6) Make sure that the visitor must register before checkout by placing the checkout pages into a Registration Folder.
Now, when a visitor adds the Product to their cart, checks out, and completes their order, their User account will be updated automatically to have access to any Pages and Files in the Private Folder.
They will click on the link in the Order Receipt Message to access the Private Page and then click on the link to download the Private File(s).