public void createPdfButton_Click(object sender, EventArgs e)
    {      
                    string brandId = pdfBrandDropDownList.SelectedValue;
                    string catId = pdfCategoryDropDownList.SelectedValue;                  

                   string strJS= "window.open('pagina.aspx?id"  + bandId + "');";
                    Page.ClientScript.RegisterClientScriptBlock(this.GetType, "OpenWindow", strJS, true);     
    }