itext Protecting your PDF

/*
 * This class is part of the book "iText in Action - 2nd Edition"
 * written by Bruno Lowagie (ISBN: 9781935182610)
 * For more info, go to: http://itextpdf.com/examples/
 * This example only works with the AGPL version of iText.
 */
 
package part3.chapter12;
 
import java.io.FileOutputStream;
import java.io.IOException;
 
import com.itextpdf.text.Document;
import com.itextpdf.text.DocumentException;
import com.itextpdf.text.Paragraph;
import com.itextpdf.text.pdf.PdfWriter;
import com.itextpdf.text.pdf.PdfReader;
import com.itextpdf.text.pdf.PdfStamper;
 
 
public class EncryptionPdf {
    /** User password. */
    public static byte[] USER = "Hello".getBytes();
    /** Owner password. */
    public static byte[] OWNER = "World".getBytes();
 
    /** The resulting PDF file. */
    public static final String RESULT1
        = "results/part3/chapter12/encryption.pdf";
    /** The resulting PDF file. */
    public static final String RESULT2
        = "results/part3/chapter12/encryption_decrypted.pdf";
    /** The resulting PDF file. */
    public static final String RESULT3
        = "results/part3/chapter12/encryption_encrypted.pdf";
 
    /**
     * Creates a PDF document.
     * @param filename the path to the new PDF document
     * @throws DocumentException 
     * @throws IOException 
     */
    public void createPdf(String filename) throws IOException, DocumentException {
        // step 1
        Document document = new Document();
        // step 2
        PdfWriter writer = PdfWriter.getInstance(document, new FileOutputStream(filename));
        writer.setEncryption(USER, OWNER, PdfWriter.ALLOW_PRINTING, PdfWriter.STANDARD_ENCRYPTION_128);
        writer.createXmpMetadata();
        // step 3
        document.open();
        // step 4
        document.add(new Paragraph("Hello World"));
        // step 5
        document.close();
    }
 
    /**
     * 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 decryptPdf(String src, String dest) throws IOException, DocumentException {
        PdfReader reader = new PdfReader(src, OWNER);
        PdfStamper stamper = new PdfStamper(reader, new FileOutputStream(dest));
        stamper.close();
        reader.close();
    }
 
    /**
     * 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 encryptPdf(String src, String dest) throws IOException, DocumentException {
        PdfReader reader = new PdfReader(src);
        PdfStamper stamper = new PdfStamper(reader, new FileOutputStream(dest));
        stamper.setEncryption(USER, OWNER,
            PdfWriter.ALLOW_PRINTING, PdfWriter.ENCRYPTION_AES_128 | PdfWriter.DO_NOT_ENCRYPT_METADATA);
        stamper.close();
        reader.close();
    }
 
    /**
     * Main method.
     *
     * @param    args    no arguments needed
     * @throws DocumentException 
     * @throws IOException
     */
    public static void main(String[] args) throws IOException, DocumentException {
        EncryptionPdf metadata = new EncryptionPdf();
        metadata.createPdf(RESULT1);
        metadata.decryptPdf(RESULT1, RESULT2);
        metadata.encryptPdf(RESULT2, RESULT3);
    }
}

http://itextpdf.com/examples/iia.php?id=219