I tried the following:

Sub RunHelpFile()
' Specifying 1 as the second argument opens the application in
' normal size and gives it the focus.
Dim RetVal
RetVal = Shell("C:\Program Files\Coax Designer II\Coax Designer
II.pdf", 1) ' Run program.
End Sub

I get an "Invalid procedure call or argument (Error 5)" error message
however.