* PDF Writer
*/
public static void writePdf(List<BufferedImage> images, ImageOutputStream ios) throws IOException {
PDFImageWriter writer = (PDFImageWriter) ImageIO.getImageWritersByFormatName("pdf").next();
writer.setOutput(ios);
writer.prepareWriteSequence(null);
for (int i=0; i < images.size(); i++) {
writer.writeToSequence(images.get(i));
}