---
title: "Serial signatures"
description: "Serial PAdES signatures in PKI SDK: each signer signs the PDF already signed by the previous one using PadesSigner, with no code changes."
canonical: "https://docs.lacunasoftware.com/en-us/articles/pki-sdk/signatures/pades/co-sign"
locale: "en"
product: "PKI SDK"
last_updated: "2026-10-12"
---

# Serial signatures

Serial signatures are the PAdES mechanism for multiple signatures. After a first signature of the PDF document, the flow
moves on to the next signer, who signs not just the document but the document with the previous signature, and so on.

A serial PAdES signature needs no changes in the SDK: just pass an already signed PDF to the next signer's signer object.
The example below makes a serial signature on an already signed PDF:

```cs
// PDF document already signed
byte[] signedPdf1 = ...

// Certificate of signer 2
PKCertificateWithKey signingCert2 = ...

var signer2 = new PadesSigner();
signer2.SetPdfToSign(signedPdf1);               // signed PDF 1
signer2.SetSigningCertificate(signingCert2);
signer2.SetPolicy(policy);
signer2.ComputeSignature();
byte[] signedPdf2 = signer2.GetPadesSignature();      // returns the signed PDF with 2 signatures
```

## See also

* @Lacuna.Pki.Pades.PadesSigner