Hi KatieNgoc,

Refer below sample code.

I have create below sample by taking reference of below link -

Zip a Folder (Directory) and Download in ASP.Net using C# and VB.Net

HTML

<asp:DropDownList runat="server" ID="ddlFiles" AutoPostBack="true" OnSelectedIndexChanged="ConverToPDF">
    <asp:ListItem Text="----Files----" Value=""></asp:ListItem>
</asp:DropDownList>
<asp:Button ID="Button1" Text="Download " runat="server" OnClick="Download" />

Namespaces

C#

using System.IO;
using iTextSharp.text;
using iTextSharp.text.html.simpleparser;
using iTextSharp.text.pdf;
using Ionic.Zip;

VB.Net

Imports System.IO
Imports iTextSharp.text
Imports iTextSharp.text.html.simpleparser
Imports iTextSharp.text.pdf
Imports Ionic.Zip

Code

C#

protected void Page_Load(object sender, EventArgs e)
{
    int i = 0;
    foreach (string filePath in Directory.GetFiles(Server.MapPath("~/HtmlFiles")))
    {
        i = i + 1;
        ddlFiles.Items.Insert(i, new System.Web.UI.WebControls.ListItem(filePath, "" + i + ""));
    }
}

protected void ConverToPDF(object sender, EventArgs e)
{
    using (StringWriter sw = new StringWriter())
    {
        using (HtmlTextWriter hw = new HtmlTextWriter(sw))
        {
            using (StreamReader sr = new StreamReader(ddlFiles.SelectedItem.Text))
            {
                using (FileStream stream = new FileStream(Server.MapPath("~/File/") + "HTMLExport" + ddlFiles.SelectedItem.Value + ".pdf", FileMode.Create))
                {
                    Document pdfDoc = new Document(PageSize.A2, 10f, 10f, 10f, 0f);
                    HTMLWorker htmlparser = new HTMLWorker(pdfDoc);
                    PdfWriter.GetInstance(pdfDoc, Response.OutputStream);
                    PdfWriter.GetInstance(pdfDoc, stream);
                    pdfDoc.Open();
                    htmlparser.Parse(sr);
                    pdfDoc.Close();
                    stream.Close();
                }
            }
        }
    }
}

protected void Download(object sender, EventArgs e)
{
    int i = 0;
    using (ZipFile zip = new ZipFile())
    {
        zip.AlternateEncodingUsage = ZipOption.AsNecessary;
        zip.AddDirectoryByName("Files");
        foreach (var filePath in Directory.GetFiles(Server.MapPath("~/HtmlFiles")))
        {
            i = i + 1;
            zip.AddFile(Server.MapPath("~/File/") + "HTMLExport" + i + ".pdf", "Files");
        }
        Response.Clear();
        Response.BufferOutput = false;
        string zipName = String.Format("Zip_{0}.zip", DateTime.Now.ToString("yyyy-MMM-dd-HHmmss"));
        Response.ContentType = "application/zip";
        Response.AddHeader("content-disposition", "attachment; filename=" + zipName);
        zip.Save(Response.OutputStream);
        Response.End();
    }
}

VB.Net

 Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)
        Dim i As Integer = 0
        For Each filePath As String In Directory.GetFiles(Server.MapPath("~/HtmlFiles"))
            i = i + 1
            ddlFiles.Items.Insert(i, New System.Web.UI.WebControls.ListItem(filePath, "" & i & ""))
        Next
    End Sub

    Protected Sub ConverToPDF(ByVal sender As Object, ByVal e As EventArgs)
        Using sw As StringWriter = New StringWriter()
            Using hw As HtmlTextWriter = New HtmlTextWriter(sw)
                Using sr As StreamReader = New StreamReader(ddlFiles.SelectedItem.Text)
                    Using stream As FileStream = New FileStream(Server.MapPath("~/File/") & "HTMLExport" + ddlFiles.SelectedItem.Value & ".pdf", FileMode.Create)
                        Dim pdfDoc As Document = New Document(PageSize.A2, 10F, 10F, 10F, 0F)
                        Dim htmlparser As HTMLWorker = New HTMLWorker(pdfDoc)
                        PdfWriter.GetInstance(pdfDoc, Response.OutputStream)
                        PdfWriter.GetInstance(pdfDoc, stream)
                        pdfDoc.Open()
                        htmlparser.Parse(sr)
                        pdfDoc.Close()
                        stream.Close()
                    End Using
                End Using
            End Using
        End Using
    End Sub

    Protected Sub Download(ByVal sender As Object, ByVal e As EventArgs)
        Dim i As Integer = 0
        Using zip As ZipFile = New ZipFile()
            zip.AlternateEncodingUsage = ZipOption.AsNecessary
            zip.AddDirectoryByName("Files")
            For Each filePath In Directory.GetFiles(Server.MapPath("~/HtmlFiles"))
                i = i + 1
                zip.AddFile(Server.MapPath("~/File/") & "HTMLExport" + i & ".pdf", "Files")
            Next
            Response.Clear()
            Response.BufferOutput = False
            Dim zipName As String = String.Format("Zip_{0}.zip", DateTime.Now.ToString("yyyy-MMM-dd-HHmmss"))
            Response.ContentType = "application/zip"
            Response.AddHeader("content-disposition", "attachment; filename=" & zipName)
            zip.Save(Response.OutputStream)
            Response.End()
        End Using
    End Sub