open file from listbox on userform

Here's the code you need for your user form.

Option Explicit

Private Sub UserForm_Initialize()
    ' 189

    With Sheet1
        ListBox1.RowSource = .Range(.Cells(2, "A"), _
                                    .Cells(.Rows.Count, "A").End(xlUp)).Address
    End With
End Sub

Private Sub ListBox1_DblClick(ByVal Cancel As MSForms.ReturnBoolean)
    ' 189

    ' wPath must end on backslash ("\")
    Const wPath     As String = "C:\Users\OSE\Desktop\11\"
    Dim wName       As String

    With ListBox1
        If .ListIndex > -1 Then
            wName = ListBox1.Value
            If LCase(Right(wName, 4)) <> ".pdf" Then wName = wName & ".pdf"
            On Error Resume Next
            WebBrowser1.Navigate wPath & wName
        Else
            MsgBox "Please select a file to display.", _
                   vbExclamation, "Invalid request"
        End If
    End With
End Sub

In the attached workbook I added a WebBrowser control to the user form. This control isn't in the Toolbox by default. To add it, right-click on the tools tab of the Toolbox, select "Additonal Controls", scroll down to "Microsoft WebBrowser" and check the item. This will add the control's icon to the Toolbox and you can now select it and add a WebBrowser control to the form like a Label or Textbox.