Here is a quick breakdown on how to change your default PDF viewer.
1. Open the Start Menu and select Settings.
2. Select "Apps" from the Settings window.
3. Select "Default Apps" from the left hand side.
4. Scroll down to the bottom of the "Default Apps" menu and select "Choose default apps by file type"
5. You will see ".pdf" in the first column, to the right of that will be an app listed, click the app (highlighted blue in the image below) and select the application you would prefer.