The "File viewer" option allows specification of the program used to view output files, and the log file. The default is "Notepad.exe".
Leaving "File viewer" blank allows the system to select the program used to open the output file. The system will use the program associated with the output file's extension. For example, for files with the .TXT extension the "Notepad" program is the default. For files with the .HTML extension a web browser will typically be used.