This function returns the installation folder of an application
that can be used by ImportAny.
The installation folder of
ImportApplicationType.ShellAssociation is null. For the Microsoft
Office applications, it will be the Microsoft Office installation
folder. If this function returns null for an application other than
ShellAssociation, it means that the application is not installed or
that ABCpdf has failed to find the application. If this is the
case, ImportAny will fail. Before
calling ImportAny, you can use this
function to make sure that ABCpdf finds the application.