Use pdf-lib to copy pages from each Puppeteer PDF into a new document. Puppeteer’s page.pdf() returns a Promise<Uint8Array>; load each byte array with PDFDocument.load(), copy its pages with copyPages(), append them to a destination document, and call save() to obtain the merged PDF bytes. Do not concatenate the buffers directly: a PDF is a structured document, not a sequence of independent files.
Install the libraries
This workflow uses Puppeteer for rendering and pdf-lib for document-level page copying. Install them in your Node.js project:
npm install puppeteer pdf-libThe Puppeteer API page cited here is labeled version 25.12.0. Your lockfile determines the version actually used, so check the installed package documentation when upgrading. The pdf-lib API reference does not establish a single package version.
The complete in-memory merge function
The function below accepts an array of PDF byte arrays and returns one Uint8Array. Pages are appended in input order: every page from the first buffer, then every page from the second, and so on.
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import { PDFDocument } from 'pdf-lib';
export async function mergePdfBuffers(pdfBuffers) {
if (!Array.isArray(pdfBuffers)) {
throw new TypeError('pdfBuffers must be an array');
}
const mergedPdf = await PDFDocument.create();
for (const pdfBuffer of pdfBuffers) {
const sourcePdf = await PDFDocument.load(pdfBuffer);
const copiedPages = await mergedPdf.copyPages(
sourcePdf,
sourcePdf.getPageIndices(),
);
for (const page of copiedPages) {
mergedPdf.addPage(page);
}
}
return mergedPdf.save();
}PDFDocument.load() accepts PDF bytes such as a Uint8Array, ArrayBuffer, or base64 input, and save() serializes the destination document as a Uint8Array according to the pdf-lib API reference. In Node.js, a Buffer extends Uint8Array, so a Buffer returned by an integration can be passed directly.
Generate the source PDFs with Puppeteer
page.pdf() renders the current page and resolves to PDF bytes. Puppeteer uses print CSS media by default. If your page is styled primarily for screens, call page.emulateMediaType('screen') before generating the PDF. Puppeteer also waits for fonts to load by default, which helps avoid capturing before web fonts are available.
import puppeteer from 'puppeteer';
import { mergePdfBuffers } from './merge-pdf-buffers.js';
import { writeFile } from 'node:fs/promises';
const browser = await puppeteer.launch();
try {
const page = await browser.newPage();
await page.goto('https://example.com/report/part-1', {
waitUntil: 'networkidle0',
});
const firstPdf = await page.pdf({
format: 'A4',
printBackground: true,
});
await page.goto('https://example.com/report/part-2', {
waitUntil: 'networkidle0',
});
const secondPdf = await page.pdf({
format: 'A4',
printBackground: true,
});
const mergedPdf = await mergePdfBuffers([firstPdf, secondPdf]);
await writeFile('report.pdf', mergedPdf);
} finally {
await browser.close();
}For separate pages, you can create a new page for each URL or reuse one page with another goto(). Keep the resulting byte arrays until the merge completes. The output of writeFile() is a normal Node.js file operation; an HTTP framework can instead send the returned bytes as its response body.
Control page order and select individual pages
Reorder whole documents
Because the outer loop determines append order, reorder the input array when a different sequence is required:
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appendixPdf,
coverPdf,
mainReportPdf,
]);This produces the appendix first, followed by the cover and main report. No sorting occurs automatically, so make the order explicit in the code that collects the buffers.
Copy selected pages
copyPages(sourcePdf, indices) accepts the source document and an array of zero-based page indices. Use it when you need only selected pages or want to interleave pages from several sources:
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import { PDFDocument } from 'pdf-lib';
export async function mergeSelectedPages(parts) {
const destination = await PDFDocument.create();
for (const part of parts) {
const source = await PDFDocument.load(part.bytes);
const pages = await destination.copyPages(source, part.pageIndices);
for (const page of pages) destination.addPage(page);
}
return destination.save();
}
const output = await mergeSelectedPages([
{ bytes: coverPdf, pageIndices: [0] },
{ bytes: chapterPdf, pageIndices: [0, 1, 2] },
{ bytes: appendixPdf, pageIndices: [1, 3] },
]);Indices must refer to pages that exist in the corresponding source. Validate them before calling the API if page selections come from user input.
Return the merged bytes from an HTTP endpoint
In a Node.js server, set a PDF content type and send the byte array using the response API provided by your framework. With a native HTTP server:
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import http from 'node:http';
http.createServer(async (_request, response) => {
try {
const bytes = await mergePdfBuffers([firstPdf, secondPdf]);
response.writeHead(200, {
'Content-Type': 'application/pdf',
'Content-Disposition': 'attachment; filename="merged.pdf"',
'Content-Length': bytes.byteLength,
});
response.end(Buffer.from(bytes));
} catch (error) {
response.writeHead(500, { 'Content-Type': 'text/plain; charset=utf-8' });
response.end('PDF merge failed');
}
}).listen(3000);Convert the returned Uint8Array to a Node Buffer only at the boundary where your framework expects one. The merge itself can remain entirely in memory.
Validate inputs before loading
An empty array, malformed PDF, or encrypted file should be treated as an input error. The cited API material does not establish a compatibility guarantee for every malformed or encrypted document, so do not assume all PDFs can be loaded and copied.
function assertPdfBytes(value, index) {
if (!(value instanceof Uint8Array) && !(value instanceof ArrayBuffer)) {
throw new TypeError(`PDF ${index} is not a byte array`);
}
}
export async function safeMerge(pdfBuffers) {
if (pdfBuffers.length === 0) {
throw new Error('At least one PDF is required');
}
pdfBuffers.forEach(assertPdfBytes);
return mergePdfBuffers(pdfBuffers);
}For a stricter application, inspect the first bytes for the conventional %PDF- signature, enforce a maximum input size, and catch the load error so a bad upload does not terminate the process. Signature checking is only an early filter; it does not prove that a file is structurally valid.
What this merge preserves—and what it may not
Page content and order
The documented operation creates a new destination document and copies source pages into it. The order in which pages are added becomes the order in the merged file.
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Interactive features and signatures
This page-copy workflow should not be advertised as preserving every specialized PDF feature. The cited pdf-lib API material does not establish behavior for digital signatures, all interactive form states, annotations, encryption, or other advanced document features. If those features matter, test the exact files and installed pdf-lib release, and verify the resulting document with the consumer that will open it.
Different page sizes
Each copied page retains its own page dimensions. A merged file can therefore contain mixed sizes and orientations. If a downstream printer or viewer requires a uniform format, normalize page sizes as a separate, deliberate transformation rather than assuming the merge will do it.
Memory, timing, and reliability considerations
In-memory cost
At merge time, the source byte arrays, loaded source documents, copied pages, and serialized output can all be live. Large reports or many parts can therefore consume substantially more memory than the sum of the final file sizes. Limit the number and size of concurrent PDF generations, close the browser in a finally block, and process jobs in bounded batches when necessary.
Generation versus merge failures
A navigation timeout or rendering failure occurs before pdf-lib is involved. Keep generation and merge errors distinguishable in logs so you know whether to retry the web page or reject a malformed PDF. Puppeteer’s default font wait and print-media behavior affect generation timing and appearance, not the page-copy algorithm.
Performance expectations
The supplied API references provide no quantitative benchmark for Puppeteer generation or pdf-lib merging. Measure with your own page sizes, fonts, images, and concurrency rather than publishing a universal throughput figure.
Troubleshoot common failures
| Symptom | Likely cause | Fix |
|---|---|---|
PDFDocument.load rejects the input |
The bytes are truncated, not a PDF, encrypted, or otherwise unsupported. | Confirm the generation call completed, check the byte type and size, save the individual buffer for inspection, and test that exact file with the installed pdf-lib version. |
| The output contains pages in the wrong order | The input array or selected indices are in an unintended order. | Log each part’s name and page indices, then reorder the array or the copyPages() index list. |
| The output is blank or styled unexpectedly | Puppeteer rendered print CSS, or navigation finished before page content was ready. | Use page.emulateMediaType('screen') when appropriate, choose a suitable waitUntil, and wait for a page-specific selector before calling page.pdf(). |
| Fonts differ between source PDFs | Fonts were unavailable at capture time or the pages intentionally use different font resources. | Ensure the page reaches its ready state; Puppeteer documents waiting for fonts by default. Check the individual PDFs before merging. |
| Process memory rises during large jobs | All source documents and the destination are retained while serialization occurs. | Reduce concurrency, split very large jobs, enforce size limits, and let references go out of scope after each completed job. |
| HTTP clients receive a corrupt download | Text encoding or a framework response helper altered binary data. | Send the bytes as binary, use application/pdf, and set Content-Length from byteLength where supported. |
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open("shot.webp", "wb").write(r.content)And in Node.js:
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FAQ
Can I merge the raw Puppeteer buffers with Buffer.concat()?
No. Concatenation produces multiple PDF byte streams, not one valid document structure. Load and copy pages into a destination PDF instead.
Do I need to write temporary files?
No. Puppeteer output, pdf-lib input, and pdf-lib output can all stay in memory. Files are needed only if your application chooses to persist or inspect them.
Are page indices zero-based?
Yes. The indices passed to copyPages() identify pages by zero-based position, so the first page is index 0.
Frequently Asked Questions
Can I merge the raw Puppeteer buffers with Buffer.concat()?
No. Concatenation does not create one valid PDF; load each buffer and copy its pages into a destination document.
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Quick Recap
Are copyPages indices zero-based?
Yes. The first page is index 0.
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