Can't compress PDF files (document scan)- iText, Java. Works just with large files

Question

I have a lot of document scans in PDF format. I have tried to compress files by using iText and PdfBox Libraries but compression works just with files larger than 1 MB. The great majority of files are a little bit smaller than 1MB.

I need some hints, books, technics or best practices for PDF optimization. Tahnks!

Actual code:

import java.awt.Graphics2D;
import java.awt.geom.AffineTransform;
import java.awt.image.BufferedImage;
import java.io.ByteArrayOutputStream;
import java.io.FileOutputStream;
import java.io.IOException;

import javax.imageio.ImageIO;

import com.itextpdf.text.DocumentException;
import com.itextpdf.text.pdf.*;
import com.itextpdf.text.pdf.parser.PdfImageObject;


    public class ResizeImage {

/** The resulting PDF file. */
//public static String RESULT = "results/part4/chapter16/resized_image.pdf";
        /** The multiplication factor for the image. */
        public static float FACTOR = 0.5f;

        /**
         * Manipulates a PDF file src with the file dest as result
         * @param src the original PDF
         * @param dest the resulting PDF
         * @throws IOException
         * @throws DocumentException
         */
        public void manipulatePdf(String src, String dest) throws IOException, DocumentException {
            PdfName key = new PdfName("ITXT_SpecialId");
            PdfName value = new PdfName("123456789");
            // Read the file
            PdfReader reader = new PdfReader(src);
            int n = reader.getXrefSize();
            PdfObject object;
            PRStream stream;
            // Look for image and manipulate image stream
            for (int i = 0; i < n; i++) {
                object = reader.getPdfObject(i);
                if (object == null || !object.isStream())
                    continue;
                stream = (PRStream)object;
                // if (value.equals(stream.get(key))) {
                PdfObject pdfsubtype = stream.get(PdfName.SUBTYPE);
                System.out.println(stream.type());
                if (pdfsubtype != null && pdfsubtype.toString().equals(PdfName.IMAGE.toString())) {
                    PdfImageObject image = new PdfImageObject(stream);
                    BufferedImage bi = image.getBufferedImage();
                    if (bi == null) continue;
                    int width = (int)(bi.getWidth() * FACTOR);
                    int height = (int)(bi.getHeight() * FACTOR);
                    BufferedImage img = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
                    AffineTransform at = AffineTransform.getScaleInstance(FACTOR, FACTOR);
                    Graphics2D g = img.createGraphics();
                    g.drawRenderedImage(bi, at);
                    ByteArrayOutputStream imgBytes = new ByteArrayOutputStream();
                    ImageIO.write(img, "JPG", imgBytes);
                    stream.clear();
                    stream.setData(imgBytes.toByteArray(), true, 9);
                    stream.put(PdfName.TYPE, PdfName.XOBJECT);
                    stream.put(PdfName.SUBTYPE, PdfName.IMAGE);
                    stream.put(key, value);
                    stream.put(PdfName.FILTER, PdfName.DCTDECODE);
                    stream.put(PdfName.WIDTH, new PdfNumber(width));
                    stream.put(PdfName.HEIGHT, new PdfNumber(height));
                    stream.put(PdfName.BITSPERCOMPONENT, new PdfNumber(8));
                    stream.put(PdfName.COLORSPACE, PdfName.DEVICERGB);
                }
            }
            // Save altered PDF
            PdfStamper stamper = new PdfStamper(reader, new FileOutputStream(dest));
            stamper.close();
            reader.close();
        }

        /**
         * Main method.
         *
         * @param    args    no arguments needed
         * @throws DocumentException
         * @throws IOException
         */
        public static void main(String[] args) throws IOException, DocumentException {
            //createPdf(RESULT);
            //new ResizeImage().manipulatePdf("src/main/resources/pdfs/PR - iText in Action - Second edition e-book_0.pdf", "src/main/resources/pdfs/compressTest.pdf");
            new ResizeImage().compressPdf("src/main/resources/pdfs/e3650495-d22f-409d-8e9a-d76b7dfed63e.pdf", "src/main/resources/pdfs/compressed.pdf");
        }

        public void compressPdf(String src, String dest) throws IOException, DocumentException {
            PdfReader reader = new PdfReader(src);
            PdfStamper stamper = new PdfStamper(reader, new FileOutputStream(dest), PdfWriter.VERSION_1_5);
            stamper.getWriter().setCompressionLevel(9);
            int total = reader.getNumberOfPages() + 1;
            for (int i = 1; i < total; i++) {
                reader.setPageContent(i, reader.getPageContent(i));
            }
            stamper.setFullCompression();
            stamper.close();
            reader.close();
        }
}