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How to Merge PDFs in Node.js (Without a 300-Line Library Setup)

#node #javascript #pdf

How to Merge PDFs in Node.js (Without a 300-Line Library Setup)

PDF merging in Node.js has a few popular approaches, and they all converge on the same frustrating place: pdf-lib, hummus, or calling out to a Python script wrapping PyPDF2.

These work. But they're not lightweight. pdf-lib is capable, but merging PDFs with complex features — embedded fonts, form fields, digital signatures — can produce broken output. And when it breaks, the error messages are not helpful.

The Setup 

npm install renderpdfs
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Merging Two PDFs 

import RenderPDFs from 'renderpdfs';

const client = new RenderPDFs('rpdf_your_key');

const merged = await client.merge([
  'https://example.com/files/contract.pdf',
  'https://example.com/files/appendix.pdf',
]);

import { writeFileSync } from 'fs';
writeFileSync('combined.pdf', merged);
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The input is an array of PDF URLs. The output is a single merged PDF Buffer.

Real Use Case: Bundling a Contract with Its Attachments 

app.post('/contracts/:id/bundle', authenticate, async (req, res) => {
  const contract = await db.contracts.findById(req.params.id);
  const attachments = await db.attachments.findByContractId(contract.id);

  const pdfUrls = [contract.pdfUrl, ...attachments.map(a => a.pdfUrl)];

  if (pdfUrls.length < 2) {
    return res.status(400).json({ error: 'Nothing to merge' });
  }

  const merged = await client.merge(pdfUrls);

  res.setHeader('Content-Type', 'application/pdf');
  res.setHeader('Content-Disposition', `attachment; filename="contract-${contract.id}-bundle.pdf"`);
  res.send(merged);
});
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Generate + Merge in One Flow 

// Step 1: Generate a cover page
const { url: coverUrl } = await client.generate({
  html: `<html><body style="font-family:sans-serif;padding:60px;text-align:center;">
    <h1 style="margin-top:200px;">${title}</h1>
    <p style="color:#555;">${subtitle}</p>
  </body></html>`,
  store: true,
});

// Step 2: Merge cover + body
const packet = await client.merge([coverUrl, existingDocumentUrl]);
res.setHeader('Content-Type', 'application/pdf');
res.send(packet);
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Error Handling 

try {
  const merged = await client.merge(urls);
} catch (err) {
  if (err.status === 422) {
    return res.status(400).json({ error: 'One or more PDF sources are invalid' });
  }
  throw err;
}
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Getting Started 

npm install renderpdfs
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100 PDFs/month free at renderpdfs.com. API docs at renderpdfs.com/docs.

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