add header on every page while dynamically generate pdf from html using iTextSharp in asp.net(C#)?

For adding header on every page of the dynamically generated pdf you need to override "PdfPageEventHelper" class's "OnStartPage" method as shown in below code.(Note:if wanted to add footer as well you can similarly override "OnEndPage" method of the same class)

public class HeaderFooter : PdfPageEventHelper
{
    public override void OnStartPage(PdfWriter writer, Document document)
    {
        var cssResolver = new StyleAttrCSSResolver();
        XMLWorkerFontProvider fontProvider =
            new XMLWorkerFontProvider(XMLWorkerFontProvider.DONTLOOKFORFONTS);
        //fontProvider.Register(HttpContext.Current.Server.MapPath("~/Tahoma Regular font.ttf"));
        CssAppliers cssAppliers = new CssAppliersImpl(fontProvider);
        HtmlPipelineContext htmlContext = new HtmlPipelineContext(cssAppliers);
        htmlContext.SetTagFactory(Tags.GetHtmlTagProcessorFactory());
        // Pipelines
        PdfWriterPipeline pdf = new PdfWriterPipeline(document, writer);
        HtmlPipeline html1 = new HtmlPipeline(htmlContext, pdf);
        CssResolverPipeline css = new CssResolverPipeline(cssResolver, html1);
        // XML Worker
        XMLWorker worker = new XMLWorker(css, true);
        XMLParser p = new XMLParser(worker);
       // you can place any Html header in the following line as string
        p.Parse(new StringReader("<div style='font-size:30px;color:#ff0000;'>Page Header</div>"));
    }
}

Now for your ease I have just updated the method you have used to generate PDF as follows.(Just updated two lines of code)

public MemoryStream GeneratePdf(string html)
        {
            var pdfDoc = new Document(PageSize.A3);
            var memoryStream = new MemoryStream();
            PdfWriter pdfWriter = PdfWriter.GetInstance(pdfDoc, memoryStream);

            pdfWriter.RgbTransparencyBlending = true;
            //following two lines updated in the previous code 
            HeaderFooter myevent = new HeaderFooter();
            pdfWriter.PageEvent = myevent; 

            pdfDoc.Open();
            var cssResolver = new StyleAttrCSSResolver();
            XMLWorkerFontProvider fontProvider =
                new XMLWorkerFontProvider(XMLWorkerFontProvider.DONTLOOKFORFONTS);
            //Following line will be required in case of lanuguages other than english i.e I have added following for arabic font.
            //fontProvider.Register(HttpContext.Server.MapPath("~/Tahoma Regular font.ttf"));
            CssAppliers cssAppliers = new CssAppliersImpl(fontProvider);
            HtmlPipelineContext htmlContext = new HtmlPipelineContext(cssAppliers);
            htmlContext.SetTagFactory(Tags.GetHtmlTagProcessorFactory());
            // Pipelines
            PdfWriterPipeline pdf = new PdfWriterPipeline(pdfDoc, pdfWriter);
            HtmlPipeline html1 = new HtmlPipeline(htmlContext, pdf);
            CssResolverPipeline css = new CssResolverPipeline(cssResolver, html1);
            // XML Worker
            XMLWorker worker = new XMLWorker(css, true);
            XMLParser p = new XMLParser(worker);
            p.Parse(new StringReader(html));
            pdfWriter.CloseStream = false;
            pdfDoc.Close();
            memoryStream.Position = 0;
            return memoryStream;
        }

this code is not work

iTextSharp.tool.xml.exceptions.RuntimeWorkerException
HResult=0x80131500
Message=
Source=itextsharp.xmlworker
StackTrace:
at iTextSharp.tool.xml.XMLWorker.EndElement(String tag, String ns)
at iTextSharp.tool.xml.parser.XMLParser.EndElement()
at iTextSharp.tool.xml.parser.state.ClosingTagState.Process(Char character)
at iTextSharp.tool.xml.parser.XMLParser.ParseWithReader(TextReader reader)
at iTextSharp.tool.xml.parser.XMLParser.Parse(TextReader reader)
at iTextSharp.tool.xml.parser.XMLParser.Parse(Stream inp)
at ConsoleApp1.Program.Main(String[] args) in C:\Users\uC254621\source\repos\ConsoleApp1\ConsoleApp1\Program.cs:line 54

This exception was originally thrown at this call stack:
[External Code]

Inner Exception 1:
PipelineException: iTextSharp.tool.xml.pipeline.html.HtmlPipeline cries, it cannot find it's own context.

Inner Exception 2:
NoCustomContextException: Exception of type 'iTextSharp.tool.xml.NoCustomContextException' was thrown.