try this:
richtextbox.Text = @"<table ID=tbtest Runat=server style=width:100%;>
<tr>
<td id=tdh" + i + "runat=server>" + myReader[i].ToString() + @"</td>
<td id=tdh2 runat=server>Pkg Type</td>
<td id=tdh3 runat=server>Description</td>
<td id=tdh4 runat=server>Demension</td>
<td id=tdh5 runat=server>Weight</td>
</tr>
<tr>
<td id=td1 runat=server>test</td>
<td id=td2 runat=server>test</td>
<td id=td3 runat=server>test</td>
<td id=td4 runat=server>test</td>
<td id=td5 runat=server>test</td>
</tr>
<tr>
<td id=td6 runat=server>test</td>
<td id=td7 runat=server>test</td>
<td id=td8 runat=server>test</td>
<td id=td9 runat=server>test</td>
<td id=tdh10 runat=server>test</td>
</tr>
</table>";