There are multiple ways of unlocking the Pro features of Highlights, to do it from a document window follow these steps:
1. Click the yellow star in the top right corner:
(Or blue button if you are on macOS 10.15 Catalina.)
This will bring up the Highlights Pro upgrade window:
2. Choose a monthly or yearly subscription
The yearly plan includes a free trial and if you cancel the subscription within the trial period, you will not be charged anything. If you are upgrading from Highlights 1 see this page.
3. Authorize the App Store purchase
The App Store will prompt you to sign in with your Apple ID. Enter your user info and click Buy or use Touch ID to authenticate.
Once the purchase is confirmed, Highlights will unlock all of the Pro features.