Create a product for the download in the normal way at admin/products.
Give it an image, price, quantity (if appropriate) etc.
First of all go into the app store, and add the free Shopify Digital Downloads app (it is free, so you shouldn’t need to be the account owner to do this).
Then you can go back in to the product , click into it, and click on the […] box in the top corner and select ‘add digital attachment’.
Add the epub etc file: