Your biggest problem here is that you cannot force users to run your macro and Windows security will conspire against you running that macro by warning users not to run it, however given that, the following macro would delete the content of the document, clear the undo stack and save the document without creating a backup.
Code:
Option Explicit
Sub Document_Open()
Dim FechaCaducidad As Date
Dim bBackup As Boolean
FechaCaducidad = #4/5/2020#
If FechaCaducidad > Date Then
'Mensaje de bienvenida
MsgBox ("Bienvenid@" + Chr(13) + Chr(10) & "LE SALUDA SU PROFESOR FELIX FALCONI" + Chr(13) + Chr(10))
Else
MsgBox "Este archivo dejo de funcionar" & vbCrLf & "Finalizó el período de utilizacion", vbCritical
Application.DisplayAlerts = False
DeleteAll
ActiveDocument.UndoClear
ActiveDocument.SaveAs ActiveDocument.FullName
End If
End Sub
Sub DeleteAll()
Dim oStory As Range
For Each oStory In ActiveDocument.StoryRanges
If oStory.StoryType = wdMainTextStory Then
oStory.Text = "Este archivo dejo de funcionar" & vbCrLf & "Finalizó el período de utilizacion"
oStory.Font.Size = 30
Else
oStory.Text = ""
End If
If oStory.StoryType <> wdMainTextStory Then
While Not (oStory.NextStoryRange Is Nothing)
Set oStory = oStory.NextStoryRange
oStory.Text = ""
Wend
End If
Next oStory
Set oStory = Nothing
lbl_Exit:
Exit Sub
End Sub