When you download Everdo for your platform, you get a fully-featured version of the app with some limits. You can use the limited version for free until you decide to upgrade.
All functionality (features) is there for you to evaluate. The only limits are:
Once a project/notebook is archived, it no longer counts as Active.
Completed projects do count as active, unless archived.
If the limits outlined above work for you, then you can use Everdo without upgrading.
Go to Upgrade and create your order. Once your order is processed, you will receive a Pro Key by email within a few minutes. After you receive the key, simply add it to Everdo via the application menu.
The product is licensed to you personally, so you can use Everdo with the same license key on multiple devices. Take care of your key though - it's a secret. If someone else wants to setup and use Everdo, a separate key is needed.