Where to place digital signature in PDF (openpgp.js)?

Question

JavaScript

Hello!
I implement a digital signature of pdf files using openpgp.js.
The question is - how / where to place the signature so that the file itself is not damaged?
If done straight:

const file = fs.readFileSync('./testdoc.pdf', 'utf-8');
let privkey = fs.readFileSync('./private1.key', 'utf-8'); //encrypted private key
let passphrase = 'secret'; //what the privKey is encrypted with
const privKeyObj = openpgp.key.readArmored(privkey).keys[0];
privKeyObj.decrypt(passphrase);

openpgp.sign({
    data: file, // input as String (or Uint8Array)
    privateKeys: privKeyObj, // for signing
    //detached: true,
})
.then(signed => {
    //console.log(signed.signature);
    fs.writeFileSync('./testdoc.pdf', signed.data);
});

Then after the signature, the file will become corrupted, as a header will appear at the beginning:

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA256

%PDF-1.3
%����
4 0 obj
<<
/Type /Page

If we make detached: true and write the signature to a separate file (which is not very good for me),
then the verification fails:

var message = openpgp.message.readSignedContent(file, sig); //file - исходный файл, без подписи, sig - файл с сигнатурой, в кодировке utf-8
var result = message.verify(openpgp.key.readArmored(pubkey).keys); // result.valid=false всегда

+ signatures can be not one but many on 1 file - how to make them independent from each other?
What is the mechanism here? Where should the signature be placed?

Answer 1

According to the PDF standard , a digital signature is stored in a special structure in the tree of the document itself.
The TCPDF library has a setSignature() method to add a signature to a PDF ( example ).