我们可以使用itext的PdfWriter类的setEncryption方法来为pdf文件设置密码

package com.open.pdf;

import java.io.File;
import java.io.FileOutputStream;
import java.io.OutputStream;
import java.util.Date;

import com.itextpdf.text.Document;
import com.itextpdf.text.Paragraph;
import com.itextpdf.text.pdf.PdfWriter;

public class GeneratePDF {

    private static String USER_PASS = "Hello123";

    private static String OWNER_PASS = "Owner123";

    public static void main(String[] args) {
        try {

            OutputStream file = new FileOutputStream(new File("D:\\Test.pdf"));

            Document document = new Document();
            PdfWriter writer = PdfWriter.getInstance(document, file);

            writer.setEncryption(USER_PASS.getBytes(), OWNER_PASS.getBytes(),
                    PdfWriter.ALLOW_PRINTING, PdfWriter.ENCRYPTION_AES_128);

            document.open();
            document.add(new Paragraph("Hello World, iText"));
            document.add(new Paragraph(new Date().toString()));

            document.close();
            file.close();

        } catch (Exception e) {

            e.printStackTrace();
        }
    }
}