PDF Viewer can directly open PDF documents shared from other apps (email clients, file browsers, chats, ...). If you tap a PDF document, for example in your email client, and a different PDF viewer automatically opens, it may be that this other app is set as default app for handling PDF documents. Here's how you can clear Google PDF viewer from being the default PDF app:
Go to Settings.
Go to Apps.
Select the other PDF app, that always open up automatically.
Scroll down to "Launch By Default" or "Open by default".
Tap "Clear Defaults" (if this button is enabled).
The next time you tap a PDF document, the app chooser should be shown, allowing you to select the PDF Viewer.