PdfDocument doc = new PdfDocument();
HtmlToPdfOptions options = new HtmlToPdfOptions();
options.PageSize = new SizeF(PdfPageSizes.A4.Width, PdfPageSizes.A4.Height);
options.AutoAdjustForDPI = true;
options.OutputArea = new RectangleF(0f, 0f, sizeF.Width, sizeF.Height);
options.GeneratePageImages = true;
string contenuHtml = "<html><body><div style=\"background-color:red;width:794px;height:1123px;\">Contenu</div></body></html>";
var result = HtmlToPdf.ConvertHtml(contenuHtml, doc, options);
for (int i = 0; i < result.PageImages.Length; i++)
{
var pageImage = result.PageImages[i];
var clonedImage = (Bitmap)pageImage.Clone();
clonedImage.SetResolution(clonedImage.HorizontalResolution * 1, clonedImage.VerticalResolution * 1);
}