After the buyer finishes checkout and the payment processor confirms the buyer's payment was good and completed, we grant the buyer access to a Thank-you Page where we present their unique download link(s) automatically, and we also issue them a Thank-you Email with a link to reach the Thank-you/download Page in case they did not proceed to the download page after checkout.
Their download link(s) expire after the number of attempts or hours (whichever comes first) that you specify in each product's settings, so even if a buyer shared their Thank-you Email or the link to their Thank-you Page with someone else, the actual download links on that page could never be valid indefinitely "out in the wild". Every valid Thank-you Page link references a specific Transaction ID, so there is no way to reach the page without using the link issued to a particular buyer.
You may want to read this forum post to get a clearer idea of the difference between a Thank-you Page and a Thank-you Email and what each is used for:
You can add custom HTML to your E-junkie thank-you pages:
If you want something to be shown the same on all thank-you pages regardless of which product(s) the buyer purchased, that would go in your Seller Admin > Account Preferences > Common Thank-you Page HTML.
If you want it to be shown on the Thank-you Page only when a specific product is purchased, that would go in the Message (HTML) field in that product's settings, on the final screen where you would click Submit to save changes and reach the button codes screen.
Redirection would only be used if you wanted to send buyers to a "secret" page at some URL after payment, instead of providing them with a file download.
The Thank-you Email for a digital product will always include the name of the product and a link to the thank-you/download page. You can add to this by typing something into the Email Message field in the product's settings.
If you want to override all that and replace our standard Thank-you Email message and link with an entirely custom message from start to finish, you can check Enable Templated Email in the product's settings, and then type your message in the Email Message field, using special template [%variables%] to insert order-specific information such as the [%download_link%] -- other variable codes are listed here:
It's generally a good idea to just use the standard Thank-you Email, or if customized to make it short and to-the-point, which reduces the chances of it being mistaken for spam. Then you can put whatever promotional text you want in the Thank-you Page.