var document = new PdfDocument();
document.Options.ColorMode = PdfColorMode.Cmyk;

using (var image = CreateImageFromByteArray(tiffData))
{
   var page = document.AddPage();
   page.Size = PageSize.A3;
   page.Orientation = PageOrientation.Portrait;

   var gfx = XGraphics.FromPdfPage(page);

   // maintain aspect ratio
   double widthScale = page.Width/image.PointWidth;
   double heightScale = page.Height/image.PointHeight;
   double scale = Math.Min(widthScale, heightScale);

   gfx.DrawImage(image, 0, 0, image.PointWidth*scale, image.PointHeight*scale);
}

private static XImage CreateImageFromByteArray(byte[] imageData)
{
   var stream = new MemoryStream(imageData);

   stream.Write(imageData, 0, imageData.Length);
   var tiffImage = Image.FromStream(stream);

   return XImage.FromGdiPlusImage(tiffImage);
}