Sub OpenAWorkbook()
Dim xlApp As Object
Dim xlBook As Object
    On Error Resume Next
    Set oFD = Application.FileDialog(msoFileDialogFilePicker)
    With oFD 'используем короткое обращение к объекту
    .Title = "Выберите файл"
    .AllowMultiSelect = False
    .ButtonName = "Выбрать"
    .Filters.Clear
    .Filters.Add "Excel files", "*.xls; *.xlsx; *.xlsm"
        If oFD.Show = 0 Then
            ActiveWindow.View.ShowFieldCodes = False
        Exit Sub 'показывает диалог
        End If
    strWorkbookName = .SelectedItems(1)
    End With

    Set xlApp = GetObject(, "Excel.Application")
    If Err Then
        Set xlApp = CreateObject("Excel.Application")
    End If
    On Error GoTo 0
    Set xlBook = xlApp.Workbooks.Open(FileName:=strWorkbookName)
    xlApp.Visible = False
    
    lLastRow = xlApp.ActiveSheet.UsedRange.Row + xlApp.ActiveSheet.UsedRange.Rows.Count - 1
    For n = 2 To lLastRow
    a = xlApp.ActiveSheet.Range("A" & n).Value
    b = xlApp.ActiveSheet.Range("B" & n).Value
    
    With ActiveDocument.Range.Find
      .Text = a
      .Replacement.Text = b
      .Execute Replace:=wdReplaceAll
    End With
    Next
    
xlApp.Workbooks.Close
'lbl_Exit:
    Set xlApp = Nothing
    Set xlBook = Nothing
    Exit Sub
End Sub