Beta apps are new, experimental versions of apps that are currently available on the Play Store. The beta version may have features that are not available in the public version yet.
To get the beta version of PDF Viewer you have to have the public version installed. Here's how you participate in the beta:
Open the Google Play Store.
Search for "PSPDFKit PDF Viewer" and tap it to go to the app detail page.
Scroll down to the very bottom of the page, as the beta information is usually located at the bottom of the page.
Tap Join to enlist in the beta. You will automatically receive all beta updates available.