Basically you would use your close macro to set the document variable e.g.
Code:
Private Sub BtnExitl_Click()
ThisDocument.Variables("varShow").value = "True"
ThisDocument.Save
Application.Quit
End Sub
and then look for that variable when you open the document again. If the variable is not present then show the userform.
Code:
Private Sub Document_Open()
Dim oVar As Variable
Dim bVar As Boolean
For Each oVar In ThisDocument.Variables
If oVar.Name = "varShow" Then
If oVar.value = "True" Then
bVar = True
Exit For
End If
End If
Next oVar
If bVar = False Then UserForm1.Show
End Sub