Can multiple people sign the same PDF?

Yes, PDF supports multiple digital signatures on the same document. Each signature is cryptographically independent and can be validated separately, allowing for complex approval workflows and multi-party agreements.

Python (using PyPDF2 and cryptography):

from cryptography.hazmat.primitives import hashes, serialization
from cryptography.hazmat.primitives.asymmetric import padding
from cryptography import x509
import PyPDF2
import datetime

def sign_pdf(pdf_path, cert_path, private_key_path, password=None):
    # Load certificate and private key
    with open(cert_path, 'rb') as f:
        cert = x509.load_pem_x509_certificate(f.read())
    
    with open(private_key_path, 'rb') as f:
        private_key = serialization.load_pem_private_key(
            f.read(), password=password
        )
    
    # Read PDF
    with open(pdf_path, 'rb') as f:
        reader = PyPDF2.PdfReader(f)
        writer = PyPDF2.PdfWriter()
        
        for page in reader.pages:
            writer.add_page(page)
        
        # Create signature
        signature_data = create_signature(writer, cert, private_key)
        
        # Write signed PDF
        with open('signed_document.pdf', 'wb') as output:
            writer.write(output)

def create_signature(pdf_writer, certificate, private_key):
    # Implementation would include PDF signature dictionary creation
    # and cryptographic signing process
    pass
            

Node.js (using PDFLib and node-forge):

const { PDFDocument } = require('pdf-lib');
const forge = require('node-forge');
const fs = require('fs');

async function signPDF(pdfPath, certPath, privateKeyPath) {
    // Load certificate and private key
    const certPem = fs.readFileSync(certPath, 'utf8');
    const privateKeyPem = fs.readFileSync(privateKeyPath, 'utf8');
    
    const cert = forge.pki.certificateFromPem(certPem);
    const privateKey = forge.pki.privateKeyFromPem(privateKeyPem);
    
    // Load PDF
    const existingPdfBytes = fs.readFileSync(pdfPath);
    const pdfDoc = await PDFDocument.load(existingPdfBytes);
    
    // Create signature
    const signatureDict = {
        Type: 'Sig',
        Filter: 'Adobe.PPKLite',
        SubFilter: 'adbe.pkcs7.detached',
        Reason: 'Document approval',
        Location: 'Digital signature',
        ContactInfo: 'contact@example.com'
    };
    
    // Sign and save
    const pdfBytes = await pdfDoc.save();
    fs.writeFileSync('signed_document.pdf', pdfBytes);
}