Click on the Windows button (bottom left hand screen) and select the gear icon for Settings.
In the "Find a settings" search box, type: default apps and select the setting.
From the Default apps window, scroll down to the bottom and select: "Choose default app by file type"
From the Choose default app by file type window, scroll down and find PDF.
Select the application to the right or, if you see Choose a default program, click that.
Select the application you wish to open the file with.