Dim tab As New TabPage With {.Text = "Pestaña" & (TabControl1.TabPages.Count + 1)}
TabControl1.TabPages.Add(tab)
 
Dim rtb As  New RichTextBox
With rtb
    .Location = New Point(6, 6)
    .AutoSize = False
    .Size = New Size(317, 393)
    .Parent = tab
    .LoadFile("C:\Archivo.rtf", RichTextBoxStreamType.RichText)
End With
 
TabControl1.SelectedTab = tab

Friend WithEvents RichTextBox1 As RichTextBox
 
Private Sub Form1_Shown(ByVal sender As Object, ByVal e As EventArgs) Handles MyBase.Shown
 
    Dim tab As New TabPage With {.Text = "Pestaña" & (TabControl1.TabPages.Count + 1)}
    TabControl1.TabPages.Add(tab)
 
    Me.RichTextBox1 = New RichTextBox
    With Me.RichTextBox1
        .Location = New Point(6, 6)
        .AutoSize = False
        .Size = New Size(317, 393)
        .Parent = tab
    End With
 
    TabControl1.SelectedTab = tab
 
End Sub
 
Private Sub Button1_Click(ByVal sender As Object, ByVal e As EventArgs) Handles Button1.Click
 
    Me.RichTextBox1.LoadFile("C:\Archivo.rtf", RichTextBoxStreamType.RichText)
 
End Sub