Office Mobile not installed yet opening PDF files
1.When searching for apps. Tap the menu  hamburger, items on the right, then tap show system files etc.
It should be in there. From this you can go into the application information and clear everything especially "Defaults".
Or
2. However, you can try this
https://play.google.com/store/apps/details?id=com.aboutmycode.betteropenwith

Or just simply install office and uninstall it after you have demonstrated step one.

Good luck