I have created an example by refering the below article.

How to export GridView data to PDF file in ASP.Net

Now please take its reference and correct your code as per your logic if required.

HTML

<asp:GridView runat="server" ID="GridView1" AutoGenerateColumns="false">
    <Columns>
        <asp:BoundField DataField="Id" HeaderText="Id" ItemStyle-Width="150px" />
        <asp:BoundField DataField="Name" HeaderText="Name" ItemStyle-Width="100px" />
        <asp:BoundField DataField="Country" HeaderText="Country" ItemStyle-Width="100px" />
    </Columns>
</asp:GridView>
<asp:Button Text="Export" runat="server" OnClick="ExportToPDF" />

Namespaces

C#

using System.Data;
using iTextSharp.text;
using iTextSharp.text.html.simpleparser;
using iTextSharp.text.pdf;

VB.Net

Imports System.Data
Imports iTextSharp.text
Imports iTextSharp.text.html.simpleparser
Imports iTextSharp.text.pdf

Code

C#

private void BindGrid()
{
    DataTable dt = new DataTable();
    dt.Columns.AddRange(new DataColumn[3] { new DataColumn("Id"), new DataColumn("Name"), new DataColumn("Fee") });
    dt.Rows.Add(1, "John Hammond", DBNull.Value);
    dt.Rows.Add(2, "Mudassar Khan", null);
    dt.Rows.Add(3, "Suzanne Mathews", "0");
    dt.Rows.Add(4, "Robert Schidner", "0");
    GridView1.DataSource = dt;
    GridView1.DataBind();
}

protected void ExportToPDF(object sender, EventArgs e)
{
    using (StringWriter sw = new StringWriter())
    {
        using (HtmlTextWriter hw = new HtmlTextWriter(sw))
        {
            GridView1.AllowPaging = false;
            this.BindGrid();
            bool isAllNullOrZero = false;
            foreach (GridViewRow row in GridView1.Rows)
            {
                if (row.Cells[2].Text == null || row.Cells[2].Text.Trim() == "0" || row.Cells[2].Text.Trim() == " ")
                {
                    isAllNullOrZero = true;
                }
                else
                {
                    isAllNullOrZero = false;
                    break;
                }
            }
            if (isAllNullOrZero)
            {
                GridView1.Columns[2].Visible = false;
            }

            GridView1.RenderControl(hw);
            StringReader sr = new StringReader(sw.ToString());
            Document pdfDoc = new Document(PageSize.A2, 10f, 10f, 10f, 0f);
            HTMLWorker htmlparser = new HTMLWorker(pdfDoc);
            PdfWriter.GetInstance(pdfDoc, Response.OutputStream);
            pdfDoc.Open();
            htmlparser.Parse(sr);
            pdfDoc.Close();
            Response.ContentType = "application/pdf";
            Response.AddHeader("content-disposition", "attachment;filename=GridViewExport.pdf");
            Response.Cache.SetCacheability(HttpCacheability.NoCache);
            Response.Write(pdfDoc);
            Response.End();
        }
    }
}

public override void VerifyRenderingInServerForm(Control control)
{
}

VB.Net

Private Sub BindGrid()
    Dim dt As DataTable = New DataTable()
    dt.Columns.AddRange(New DataColumn(2) {New DataColumn("Id"), New DataColumn("Name"), New DataColumn("Fee")})
    dt.Rows.Add(1, "John Hammond", DBNull.Value)
    dt.Rows.Add(2, "Mudassar Khan", Nothing)
    dt.Rows.Add(3, "Suzanne Mathews", "0")
    dt.Rows.Add(4, "Robert Schidner", "0")
    GridView1.DataSource = dt
    GridView1.DataBind()
End Sub

Protected Sub ExportToPDF(ByVal sender As Object, ByVal e As EventArgs)
    Using sw As StringWriter = New StringWriter()
        Using hw As HtmlTextWriter = New HtmlTextWriter(sw)
            GridView1.AllowPaging = False
            Me.BindGrid()
            Dim isAllNullOrZero As Boolean = False
            For Each row As GridViewRow In GridView1.Rows
                If row.Cells(2).Text Is Nothing OrElse row.Cells(2).Text.Trim() = "0" OrElse row.Cells(2).Text.Trim() = " " Then
                    isAllNullOrZero = True
                Else
                    isAllNullOrZero = False
                    Exit For
                End If
            Next

            If isAllNullOrZero Then
                GridView1.Columns(2).Visible = False
            End If

            GridView1.RenderControl(hw)
            Dim sr As StringReader = New StringReader(sw.ToString())
            Dim pdfDoc As Document = New Document(PageSize.A2, 10F, 10F, 10F, 0F)
            Dim htmlparser As HTMLWorker = New HTMLWorker(pdfDoc)
            PdfWriter.GetInstance(pdfDoc, Response.OutputStream)
            pdfDoc.Open()
            htmlparser.Parse(sr)
            pdfDoc.Close()
            Response.ContentType = "application/pdf"
            Response.AddHeader("content-disposition", "attachment;filename=GridViewExport.pdf")
            Response.Cache.SetCacheability(HttpCacheability.NoCache)
            Response.Write(pdfDoc)
            Response.[End]()
        End Using
    End Using
End Sub

Public Overrides Sub VerifyRenderingInServerForm(ByVal control As Control)
End Sub

Screenshot