To change the viewer default to Xodo, do the following:

1. Open the device's settings (via the Settings app, or via the gear at the top of the fully expanded notification pulldown)
2. Under the heading "Device", tap the "Apps" entry.
3. In the list of apps, tap the one that currently opens the files by default.
4. Tap the entry "Open by default".
5. Under the heading "Other defaults", tap the button "Clear Defaults".

Next time you open a file that Xodo can open, you will be presented with a choice of apps, including Xodo. Select Xodo, and then tap "Always". From now on, files of this type will open automatically using Xodo.