Microsoft.VisualBasic.Shell("C:\test.pdf", AppWinStyle.NormalFocus, True)

Declare Function ShellExecute Lib "shell32.dll" _
Alias "ShellExecuteA" (ByVal hwnd As Integer, ByVal _
lpOperation As String, ByVal lpFile As String, ByVal _
lpParameters As String, ByVal lpDirectory As String, ByVal _
nShowCmd As Long) As Long
 
    Sub OpenPDF()
        Dim PDFFile As String
        PDFFile = Application.StartupPath + "Readme.pdf"
        ShellExecute(1000, "open", PDFFile, "", "", 1)
 
    End Sub