Private Sub Document_New()
Application.ScreenUpdating = False
Dim InvoiceFile As String, InvNum As String
InvoiceFile = Options.DefaultFilePath(wdDocumentsPath) = "N:\Engineering\ECN & ER Forms" & "\NUMBERS.ini"
InvNum = System.PrivateProfileString(InvoiceFile, "InvoiceNumber", "InvNum")
'If there is no InvoiceNumber reference in the ini file
'Create one and set the number to 1, otherwise increment the number
If InvNum = "" Then
  InvNum = 1
Else
  InvNum = InvNum + 1
End If
System.PrivateProfileString(InvoiceFile, "InvoiceNumber", "InvNum") = InvNum
With ActiveDocument
  'Update the value stored in the document property
  .CustomDocumentProperties("InvNum") = InvNum
  'Update the fields in the document
  .Fields.Update
End With
Application.ScreenUpdating = True
End Sub