VBA for multiple workbooks in a directory

If you want to apply all excel files ( including closed );

Sub ProcessFiles()
    Dim Filename, Pathname As String
    Dim wb As Workbook

    Pathname = ActiveWorkbook.Path & "\Files\"
    Filename = Dir(Pathname & "*.xls")
    Do While Filename <> ""
        Set wb = Workbooks.Open(Pathname & Filename)
        DoWork wb
        wb.Close SaveChanges:=True
        Filename = Dir()
    Loop
End Sub

Sub DoWork(wb As Workbook)
    With wb
        'Do your work here
        .Worksheets(1).Range("A1").Value = "Hello World!"
    End With
End Sub