read file from sharepoint 2013 _layouts folder

i want to get the path of a file from _layouts folder in sharepoint 2013, for a stream input.

i tried:

1-
        iTextSharp.text.Document doc = new iTextSharp.text.Document(PageSize.A4);

        var path = Server.MapPath(@"\15\TEMPLATE\LAYOUTS\wfcourrierdep\images
        \LettreI.png");

          var output = new FileStream(path, FileMode.Create);
          var writer = PdfWriter.GetInstance(doc, output);

          doc.Open();

  i got an error

2-
           iTextSharp.text.Document doc = new iTextSharp.text.Document(PageSize.A4);

           var path = SPUtility.GetCurrentGenericSetupPath(@"TEMPLATE\LAYOUTS
           \wfcourrierdep\pdf\Dictionnaire.pdf");

            var output = new FileStream(path, FileMode.Create);
            var writer = PdfWriter.GetInstance(doc, output);

            doc.Open();

  i got an error

i want to use the GetCurrentGenericSetupPath or GetVersionedGenericSetupPath but i don't know what is the format of my url i should pass in. 

thanks a lot for your helps