Sub AddRandomBorder()
  Dim aRng As Range, i As Integer, aPara As Paragraph, aShp As Shape
  Dim iWidth As Integer, iHeight As Integer
  
  With ActiveDocument.Sections(1).PageSetup
    iWidth = .PageWidth - 40
    iHeight = .PageHeight - 40
  End With
  'Get rid of the existing borders
  For i = ActiveDocument.Shapes.Count To 1 Step -1
    If ActiveDocument.Shapes(i).AlternativeText = "Page Border" Then ActiveDocument.Shapes(i).Delete
  Next i
  
  For Each aPara In ActiveDocument.Paragraphs
    Set aRng = aPara.Range
    aRng.Collapse Direction:=wdCollapseStart
    If aRng.Information(wdActiveEndPageNumber) > i Then
      Set aShp = ActiveDocument.Shapes.AddShape(Type:=1, Left:=20, Top:=20, _
              Width:=iWidth, Height:=iHeight, Anchor:=aRng)
      With aShp
        .AlternativeText = "Page Border"
        .Line.ForeColor = RGB(Rnd() * 255, Rnd() * 255, Rnd() * 255)
        .Line.Weight = 3
        .Fill.Transparency = 1
      End With
      i = aRng.Information(wdActiveEndPageNumber)
    End If
  Next aPara
End Sub