PDFファイルの文字画像だけを切り取り。

using (var pdfDocument = PdfReader.Open("input.pdf", PdfDocumentOpenMode.ReadOnly))
{
  for (var i = 0; i < pdfDocument.PageCount; i++)
  {
    var page = pdfDocument.Pages[i];
    var images = page.GetImages()
      .Where(x => x.ColorSpace.Name == "DeviceGray" || x.ColorSpace.Name == "DeviceCMYK");
    foreach (var image in images)
    {
      using (var pngStream = new MemoryStream())
      {
        var png = Image.GetInstance(image);
        png.Save(pngStream, ImageFormat.Png);
        var content = new Content()
        {
          FileName = $"{i}_{image.GetHashCode()}.png",
          ContentType = "image/png",
          Content = pngStream.ToArray()
        };
        yield return content;
      }
    }
  }
}