Java实现修改PDF文件MD5值且保持内容不变

import org.apache.pdfbox.pdmodel.PDDocument;
import org.apache.pdfbox.pdmodel.PDDocumentInformation;

import java.io.File;

public class MetadataChanger {
    public static void main(String[] args) throws Exception {
        File file = new File("original.pdf");
        PDDocument document = PDDocument.load(file);

        PDDocumentInformation info = document.getDocumentInformation();
        info.setAuthor("Updated Author");
        info.setTitle("Updated Title");

        document.save("updated.pdf");
        document.close();
    }
}

import com.itextpdf.text.*;
import com.itextpdf.text.pdf.*;

import java.io.FileInputStream;
import java.io.FileOutputStream;

public class WatermarkAdder {
    public static void main(String[] args) throws Exception {
        PdfReader reader = new PdfReader(new FileInputStream("original.pdf"));
        PdfStamper stamper = new PdfStamper(reader, new FileOutputStream("watermarked.pdf"));

        Font font = new Font(Font.FontFamily.HELVETICA, 36, Font.BOLD, new BaseColor(255,255,255,0));
        Phrase phrase = new Phrase("Watermark", font);

        for (int i = 1; i <= reader.getNumberOfPages(); i++) {
            PdfContentByte over = stamper.getOverContent(i);
            ColumnText.showTextAligned(over, Element.ALIGN_CENTER, phrase, 297, 421, 45);
        }

        stamper.close();
        reader.close();
    }
}

import java.util.zip.*;
import java.io.*;

public class ZipModifier {
    public static void main(String[] args) throws IOException {
        File source = new File("original.pdf");
        File destination = new File("modified.zip");

        try (FileOutputStream fos = new FileOutputStream(destination);
             ZipOutputStream zos = new ZipOutputStream(fos)) {
             
            ZipEntry entry = new ZipEntry(source.getName());
            zos.putNextEntry(entry);
            
            byte[] buffer = new byte[1024];
            try (FileInputStream fis = new FileInputStream(source)) {
                int length;
                while ((length = fis.read(buffer)) > 0) {
                    zos.write(buffer, 0, length);
                }
            }

            zos.setComment("New Comment");
            zos.closeEntry();
        }
    }
}