The following VB6 code will open an external file with it’s associated application. This assumes that there is an application on the system that is already associated with a specific executable.
Under your General Declarations:
‘ Runs an external file/launches it’s associated app
Private Declare Function ShellExecute Lib "shell32.dll" Alias "ShellExecuteA" (ByVal hWnd As Long, ByVal lpOperation As String, ByVal lpFile As String, ByVal lpParameters As String, ByVal lpDirectory As String, ByVal nShowCmd As Long) As Long
On whatever Form/Button/Event
Dim FileName As String
FileName = "C:\path\to\some\file.pdf"
ShellExecute 0&, "open", FileName, "", "", vbNormalFocus