You should use the ShellExecute Api to open a file with it's default program:

Code:
'Put this in a module

Public 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

'Open a file with it's default program
Public Sub Execute(file As String)
    lngResult = ShellExecute(hWnd, "Open", file, "", "", vbNormalFocus)
End Sub

'In a form:
Call Execute("c:\MyFile.pdf")
have fun!