The Premium key won’t show up in the Play store anymore since it’s been discontinued (as of March 2022).
You need to go to the Google Play store and click on the upper right to get into your account (you will see a letter in a circle – click there). You will then see Purchases and Subscriptions and then Budget & History. Under there, you will see Apps that you either downloaded for free or purchased. You will see Docs To Go Premium Key listed and then you can tap on that to download it.