Fail to load PDF after conversion from HTML to PDF using htmlagilitypack

Question

I am having trouble converting an HTML to PDF using htmlagilitypack. The file being uploaded to the server cannot be opened using internet browsers.

Below are my codes.

private string GetPdfDir()
{
    var pdfDir = Ctx.Server.MapPath("~/pdf");

    if (!Directory.Exists(pdfDir))
        Directory.CreateDirectory(pdfDir);

    return pdfDir;
}

public async Task<Result> HtmlToPDF() => await TaskResult(r =>
{
    var htmltopdf = Q["Body"].ToString(); //-----> this is my parsed DOM html

    HtmlDocument doc = new HtmlDocument();

    doc.LoadHtml(htmltopdf);

    var fileName = "Checkout-" + DateTime.Now.ToString("yyyyMMdd_hhmm") + ".pdf";

    var pdfDir = GetPdfDir();

    var pdfFile = Path.Combine(pdfDir, fileName);

    doc.Save(pdfFile);

    r.ResultSet = new { FileName = fileName };

    return r;
});