Not sure why you'd want to open the file in a specific application but the best way to open a file using the associated program is
System.Diagnostics.Process.Start("PathToYourFile");
If you really, REALLY know what you are doing and want to open it in a specific application, it will most likely support command line arguments.
System.Diagnostics.Process.Start("ApplicationPath", "PathToYourFile");
Cheers, Vıkram.
After all is said and done, much is said and little is done.