Changing MIME Type

First check the MIME type of the file in question:

mimetype -b the-file.xyz

You’ll get something like application/pdf. Keep it in mind and open ~/.config/mimeapps.list.

In [Default Applications] add:

[Default Applications]
application/pdf=pdfviewer.desktop

To confirm, you can run this command on a file for which you changed the default application:

xdg-mime query default $(mimetype -b the-file.xyz)