Risolto, utilizzo l'oggetto Matrix al posto di PdfImageObject:

codice:
public class ImageExtractionStrategy : ITextExtractionStrategy
{
  public List<Matrix> imagesMatrix = new List<Matrix>();
  ...
  public void RenderImage(ImageRenderInfo renderInfo) 
  {
    Matrix matrix = renderInfo.GetImageCTM();
    if (matrix == null)
      return;
    imagesMatrix.Add(matrix);
  }
}
Utilizzo:
codice:
foreach (Matrix matrix in ImageExtractionStrategy.imagesMatrix)
{
  float imageWidth = PdfUtilMethod.PtToMM(matrix[Matrix.I11]);
  float imageHeight = PdfUtilMethod.PtToMM(matrix[Matrix.I22]);
  float imageX = PdfUtilMethod.PtToMM(matrix[Matrix.I31]);
  float imageY = PdfUtilMethod.PtToMM(matrix[Matrix.I32]);
}