Command behind a GridView ButtonField

Question

I got a question about the command behind a ButtonField (image type) in a GridView.

Got a GridView called gvIngevuld in wich I show rows of data and 1 ButtonField row. Now I have to put code behind these buttons (Each of them the same) to put some things in PDF format. The problem is I don't know how to put code behind these buttons ? The code I have :

<asp:ButtonField ButtonType="Image" ImageUrl="~/Images/pdf.png"  CommandName="pdf_click" />

As you can see I used the CommandName="pdf_click" property but when I click one of the buttons there's only a postback but nothing happens. Anyone who can help me out here ?

in case needed, code behind :

protected void pdf_click(object sender, EventArgs e)
{
    lblWelkom.Text = "Succes!";
}

Thanks.