Encrypt Unprotected PDF with Password using iText in Java

In this tutorial we will learn how to use iText API to add password in PDF and how to encrypt existing PDF to write and read data respectively.  While creating password protected PDF, we need to add user password, owner password, permission and encryption type. To encrypt existing PDF with password, iText provides PdfStamper.setEncryption which accepts same arguments as PdfWriter.setEncryption

/**
*
* @author mahsin
*/
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import com.itextpdf.text.DocumentException;
import com.itextpdf.text.pdf.PdfReader;
import com.itextpdf.text.pdf.PdfStamper;
import com.itextpdf.text.pdf.PdfWriter;
public class Main {
public static void main(String[] args) throws FileNotFoundException, DocumentException, IOException {
       PdfReader reader = new PdfReader("C:/PasswordNOTGivenPDF.pdf");
       PdfStamper stamper = new PdfStamper(reader, new FileOutputStream("C:/PDFwithPassword.pdf"));
       stamper.setEncryption("password".getBytes(), "owner_password".getBytes(),PdfWriter.ALLOW_COPY, PdfWriter.ENCRYPTION_AES_256);
       stamper.close();
       reader.close();
       System.out.println("Successfully Done");
}
}