The following example shows a Quest that have configured a discount of 10% for the Sample Download.
This quest only can be unlocked by visiting the EDD Discounts Demo.
Earning this Quest you will get a discount of 10% on Sample Download
If you try to purchase the download now, you will get the discount applied to your cart automatically.
Additionally, you can see that the discount will get displayed on the shop as well as on the single download page.
EDD Discounts add-ons offers a great number of options like apply the discount to all downloads and also to limit the number of uses (globally and/or per user).