Adjust settings directly after installation
When opened for the first time, Letter Opener runs the setup assistant to help setting up how files will be opened and displayed.
If you want to run the setup assistant again just go to Letter Opener > Setup Assistant
Change settings manually
You can set the default calendar app, PDF app and email app as well as the reader you prefer to view messages with and to where converted files should be saved.
Simply go to Letter Opener > Preferences…