Using drawText("2023-05-01T15"), the PDF output changes every time, from same input file. Why?

[bluepuma77]

We load a PDF every 5 minutes, add a hourly timestamp (like "2023-05-01T15"), so file should only change every hour.

But the output is different every single time, even though we add the same data.

Are there timestamps saved to the PDF? How can we avoid that, so same change creates byte-same output?

[bluepuma77]

I created a little script to test this:

const fs = require('fs');
const PDFDocument = require('pdfkit');
const { PDFDocument: PDFDocument2, rgb } = require('@cantoo/pdf-lib');

function createPDF(output) {
    console.log('createPDF()', output)
    const doc = new PDFDocument();
    doc.pipe(fs.createWriteStream(output));
    doc.end();
}

function updatePDF(input, output) {
    console.log('updatePDF()', input, output)
    const bytes = fs.readFileSync(input);
    PDFDocument2.load(bytes)
      .then((doc) => {
        const pages = doc.getPages();
        const firstPage = pages[0];
        firstPage.drawText('hello', { x: 50, y: 50, size: 50, color: rgb(0, 0, 0) });
        const bytes = doc.save()
          .then((data) => fs.writeFileSync(output, data));
      })
      .catch((err) => console.log(err));
}

function compareFile(file1, file2) {
  console.log('compareFile()', file1, file2)
  const bytes1 = fs.readFileSync(file1);
  const bytes2 = fs.readFileSync(file2);

  let count = 0
  for (let i = 0; i < bytes1.length; i++) {
    if (bytes1[i] !== bytes2[i]) {
      count++
      //console.log(`Byte ${i} is different`, bytes1[i], bytes2[i]);
    }
  }
  console.log('Bytes different:', count)
}

createPDF('test.pdf')
setTimeout(updatePDF, 1000, 'test.pdf', 'test1.pdf')
setTimeout(updatePDF, 2000, 'test.pdf', 'test2.pdf')
setTimeout(compareFile, 3000, 'test1.pdf', 'test2.pdf')

[bluepuma77]

% node test.js
createPDF test.pdf
updatePDF() test1.pdf
updatePDF() test2.pdf
compareFile() test1.pdf test2.pdf
Bytes different: 59

% node test.js
createPDF test.pdf
updatePDF() test1.pdf
updatePDF() test2.pdf
compareFile() test1.pdf test2.pdf
Bytes different: 413

% node test.js
createPDF test.pdf
updatePDF() test1.pdf
updatePDF() test2.pdf
compareFile() test1.pdf test2.pdf
Bytes different: 356

% node test.js
createPDF test.pdf
updatePDF() test1.pdf
updatePDF() test2.pdf
compareFile() test1.pdf test2.pdf
Bytes different: 58

% node test.js
createPDF test.pdf
updatePDF() test1.pdf
updatePDF() test2.pdf
compareFile() test1.pdf test2.pdf
Bytes different: 399

[bluepuma77]

In the source code I found the modified timestamp.

This did not work for me, the file was still different every time:

const pdfDoc = await PDFDocument.load()
const pages = pdfDoc.getPages()
const mod = pdfDoc.getModificationDate() || new Date()
pages[0].drawText('HELLO', { x: 200 })
pdfDoc.setModificationDate(mod)
const pdfBytes = Buffer.from(await pdfDoc.save())

This did work, file always has the same checksum:

const pdfDoc = await PDFDocument.load()
const pages = pdfDoc.getPages()
pages[0].drawText('HELLO', { x: 200 })
pdfDoc.setModificationDate(pdfDoc.getCreationDate() || new Date())
const pdfBytes = Buffer.from(await pdfDoc.save())