Public Class Form1
    Dim xlslist, pdflist, restlist As New List(Of String)
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
      System.EventArgs) Handles Button1.Click
        GetFiles(TextBox1.Text)
    End Sub
    Private Sub GetFiles(ByVal path As String)
        If IO.Directory.Exists(path) Then
            Dim glist As New List(Of String)
            xlslist.Clear() : ListBox1.Items.Clear()
            pdflist.Clear() : ListBox2.Items.Clear()
            restlist.Clear() : ListBox3.Items.Clear()
            glist.AddRange(IO.Directory.GetFiles(path, "*.*", _
              IO.SearchOption.AllDirectories))
            For Each s As String In glist
                Select Case IO.Path.GetExtension(s)
                    Case ".xls" : xlslist.Add(s) : ListBox1.Items.Add( _
                      IO.Path.GetFileName(s))
                    Case ".pdf" : pdflist.Add(s) : ListBox2.Items.Add( _
                    IO.Path.GetFileName(s))
                    Case Else : restlist.Add(s) : ListBox3.Items.Add( _
                    IO.Path.GetFileName(s))
                End Select
            Next
        End If
    End Sub
 
    Private Sub ListBox_MouseDoubleClick(ByVal sender As Object, ByVal e As _
      System.Windows.Forms.MouseEventArgs) Handles _
    ListBox1.MouseDoubleClick, ListBox2.MouseDoubleClick, _
    ListBox3.MouseDoubleClick
        Dim lb As ListBox = CType(sender, ListBox)
        If lb.IndexFromPoint(e.X, e.Y) > -1 Then
            Select Case lb.Name
                Case "ListBox1" : Process.Start(xlslist(lb.IndexFromPoint(e.X, _
                  e.Y)))
                Case "ListBox2" : Process.Start(pdflist(lb.IndexFromPoint(e.X, _
                e.Y)))
                Case "ListBox3" : Process.Start(restlist(lb.IndexFromPoint(e.X, _
                e.Y)))
            End Select
        End If
    End Sub
End Class