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>Also, just as a test, I specified non-existing .pdf file. But ShellExecute() didn't create a run-time error. Is it by design? How can I trap if user does not specify a valid location/name of a PDF file?

Why would it generate run-time error if it returns code indicating failure or success? The return code < 32 indicates error. In addition you can use FILE() or ADIR() functions to check for file existence even before launching the ShellExecute().