@ChrisGr Do you get asked which app to use, when opening a PDF? If not, you need to "unregister" the current one.

Open Settings, then go to Apps. Open the app the is currently registered. When you scroll down there should be a section with "standard actions" (sorry, I use a German OS) with a button "Clear standard settings" (or something similar).

Once you have done that, the next time you open a PDF, you will be asked which app to use.