Change default PDF App

On the desktop of the screen please right click anyone of the PDF's that you wish to open with a different app. This might be shown as an icon different to the default as shown below. The icon in this case is shown below as Google Chrome as the default app has been changed. Once you have right clicked the icon then select "Open With" and then select "Choose another app"

On the next page select the desired app and tick the box  to always use this app. Click "Ok" and all the files associated with the app will automatically update and open with the desired app selected.