If you are certain that a specific viewing application is present on the system on which your application will run, you can use the IDL SPAWN procedure to display a help file using that application.
Note that you must have some fairly explicit information about the system on which your application will run to use this technique. You must know:
(If your application is complex enough to have an installation program or procedure, you might be able to query the user for the path to the external viewer at installation time.)
For example, suppose you know that your application will run on a Windows system, you could open a text file in the Notepad application, which is always located in the Windows system directory and can be invoked without specifying a full path:
SPAWN, 'notepad.exe D:/myapp/myfile.txt', /NOSHELL, /NOWAIT
For more information, see SPAWN.