Try this code..
DataTable dsInput = new DataTable();
dsInput.Columns.Add("PNO");
dsInput.Columns.Add("Descr");
dsInput.Rows.Add("12", "AB");
dsInput.Rows.Add("13", "21");
dsInput.Rows.Add("12", "CD");
dsInput.Rows.Add("13", "14");
DataTable dsOutput = new DataTable();
dsOutput.Columns.Add("PNO");
dsOutput.Columns.Add("Descr");
string[] distinctPNO = dsInput.Rows.OfType<datarow>().Select(k => k["PNO"].ToString()).Distinct().ToArray();
foreach (string PNO in distinctPNO)
{
string concat = "";
var rows = dsInput.Select("PNO='" + PNO + "'");
rows.Select(k => k["Descr"].ToString()).ToList().ForEach(k => { concat += k + "/"; });
dsOutput.Rows.Add(PNO, concat.TrimEnd('/'));
}</datarow>