Hi Fahimahmed, 

 Pleaser refer below sample.

HTML Page

<img src="https://www.aspsnippets.com/images/Blue/Logo.png" alt="Alternate Text" />
<br />
<br />
<div>
    <div style="border-top: 3px solid #22BCE5">
        &nbsp;
    </div>
    <span class="btn btn-info">
        Hello <b>{UserName}</b>
        <br /><br />
        A new article has been published on ASPSnippets.
        <br />
        <a style="color: #22BCE5" href="{Url}">{Title}</a><br />
        {Description}
        <br /><br />
        Thanks<br />
        ASPSnippets
    </span>
</div>

HTML

<asp:Button Text="Export" ID="btnExport" runat="server" OnClick="Export" />

Namespaces

C#

using System.IO;
using System.Text;
using iTextSharp.text;
using iTextSharp.text.pdf;
using iTextSharp.tool.xml.parser;
using iTextSharp.tool.xml;
using iTextSharp.tool.xml.pipeline.css;
using iTextSharp.tool.xml.pipeline.html;
using iTextSharp.tool.xml.html;
using iTextSharp.tool.xml.pipeline.end;

VB.Net

Imports System.IO
Imports System.Text
Imports iTextSharp.text
Imports iTextSharp.text.pdf
Imports iTextSharp.tool.xml.parser
Imports iTextSharp.tool.xml
Imports iTextSharp.tool.xml.pipeline.css
Imports iTextSharp.tool.xml.pipeline.html
Imports iTextSharp.tool.xml.html
Imports iTextSharp.tool.xml.pipeline.end

Code

C#

protected void Export(object sender, EventArgs e)
{
    string body = string.Empty;
    using (StreamReader reader = new StreamReader(Server.MapPath("~/EmailTemplate.html")))
    {
        body = reader.ReadToEnd();
    }
    body = body.Replace("{UserName}", "Mudassar");
    body = body.Replace("{Title}", "Convert HTML File to PDF using C# and VB.Net in ASP.Net");
    body = body.Replace("{Url}", "https://www.aspsnippets.com/Articles/Send-Email-using-HTML-Templates-in-ASPNet-with-C-and-VBNet.aspx");
    body = body.Replace("{Description}", "Here Mudassar Ahmed Khan has explained with an example, how to send Email using HTML Templates in ASP.Net with C# and VB.Net.");
    byte[] byteArray = Encoding.UTF8.GetBytes(body);
    using (StringWriter sw = new StringWriter())
    {
        using (HtmlTextWriter hw = new HtmlTextWriter(sw))
        {
            using (StreamReader sr = new StreamReader(new MemoryStream(byteArray)))
            {
                Document pdfDoc = new Document(PageSize.A2, 80f, 80f, 80f, 80f);
                PdfWriter PdfWriter = PdfWriter.GetInstance(pdfDoc, Response.OutputStream);
                HtmlPipelineContext htmlContext = new HtmlPipelineContext(null);
                htmlContext.SetTagFactory(Tags.GetHtmlTagProcessorFactory());
                ICSSResolver cssResolver = XMLWorkerHelper.GetInstance().GetDefaultCssResolver(false);
                cssResolver.AddCssFile("https://maxcdn.bootstrapcdn.com/bootstrap/3.3.2/css/bootstrap.min.css", true);
                IPipeline pipeline = new CssResolverPipeline(cssResolver, new HtmlPipeline(htmlContext, new PdfWriterPipeline(pdfDoc, PdfWriter)));
                var worker = new XMLWorker(pipeline, true);
                var xmlParse = new XMLParser(true, worker);
                pdfDoc.Open();
                xmlParse.Parse(sr);
                xmlParse.Flush();
                pdfDoc.Close();
                Response.ContentType = "application/pdf";
                Response.AddHeader("content-disposition", "attachment;filename=HTMLExport.pdf");
                Response.Cache.SetCacheability(HttpCacheability.NoCache);
                Response.Write(pdfDoc);
                Response.End();
            }
        }
    }
}

VB.Net

Protected Sub Export(ByVal sender As Object, ByVal e As EventArgs)
    Dim body As String = String.Empty
    Using reader As StreamReader = New StreamReader(Server.MapPath("~/EmailTemplate.html"))
        body = reader.ReadToEnd()
    End Using
    body = body.Replace("{UserName}", "Mudassar")
    body = body.Replace("{Title}", "Convert HTML File to PDF using C# and VB.Net in ASP.Net")
    body = body.Replace("{Url}", "https://www.aspsnippets.com/Articles/Send-Email-using-HTML-Templates-in-ASPNet-with-C-and-VBNet.aspx")
    body = body.Replace("{Description}", "Here Mudassar Ahmed Khan has explained with an example, how to send Email using HTML Templates in ASP.Net with C# and VB.Net.")
    Dim byteArray As Byte() = Encoding.UTF8.GetBytes(body)
    Using sw As StringWriter = New StringWriter()
        Using hw As HtmlTextWriter = New HtmlTextWriter(sw)
            Using sr As StreamReader = New StreamReader(New MemoryStream(byteArray))
                Dim pdfDoc As Document = New Document(PageSize.A2, 80.0F, 80.0F, 80.0F, 80.0F)
                Dim PdfWriter As PdfWriter = PdfWriter.GetInstance(pdfDoc, Response.OutputStream)
                Dim htmlContext As HtmlPipelineContext = New HtmlPipelineContext(Nothing)
                htmlContext.SetTagFactory(Tags.GetHtmlTagProcessorFactory())
                Dim cssResolver As ICSSResolver = XMLWorkerHelper.GetInstance().GetDefaultCssResolver(False)
                cssResolver.AddCssFile("https://maxcdn.bootstrapcdn.com/bootstrap/3.3.2/css/bootstrap.min.css", True)
                Dim pipeline As IPipeline = New CssResolverPipeline(cssResolver, New HtmlPipeline(htmlContext, New PdfWriterPipeline(pdfDoc, PdfWriter)))
                Dim worker = New XMLWorker(pipeline, True)
                Dim xmlParse = New XMLParser(True, worker)
                pdfDoc.Open()
                xmlParse.Parse(sr)
                xmlParse.Flush()
                pdfDoc.Close()
                Response.ContentType = "application/pdf"
                Response.AddHeader("content-disposition", "attachment;filename=HTMLExport.pdf")
                Response.Cache.SetCacheability(HttpCacheability.NoCache)
                Response.Write(pdfDoc)
                Response.End()
            End Using
        End Using
    End Using
End Sub

 Screenshot