How to merge PDFs using JavaScript

In this post, you’ll learn how to combine multiple PDF files using our merge PDF JavaScript API. With our API, you receive 200 credits with the free plan. Different operations on a document consume different amounts of credits, so the number of PDF documents you can generate may vary. You’ll just need to create a free account(opens in a new tab) to get access to your API key.

Nutrient DWS Processor API

Document merging is just one of our 30+ PDF API tools. You can combine our merging tool with other tools to create complex document processing workflows, such as:

• Converting MS Office files and images into PDFs before merging
• Performing OCR on several documents before merging
• Merging, watermarking, and flattening PDFs

Once you create your account, you’ll be able to access all our PDF API tools.

Step 1 — Creating a free account on Nutrient

Go to our website(opens in a new tab), where you’ll see the page below, prompting you to create your free account.

Once you’ve created your account, you’ll be welcomed by the page below, which shows an overview of your plan details.

As you can see in the bottom-left corner, you’ll start with 200 credits to process, and you’ll be able to access all our PDF API tools.

Copy the Live API key, because you’ll need this for the merge PDF API.

Step 2 — Setting up files and folders

Now, create a folder called merge_pdf and open it in a code editor. For this tutorial, you’ll use VS Code as your primary code editor. Next, create two folders inside merge_pdf and name them input_documents and processed_documents.

Then, in the root folder, merge_pdf, create a file called processor.js. This is where you’ll keep your code.

Step 3 — Installing dependencies

To get started merging PDF pages, you first need to install the following dependencies:

• axios(opens in a new tab) — This package is used for making REST API calls.
• Form-Data(opens in a new tab) — This package is used for creating form data.

Use the commands below to install both of them:

npm install form-data

npm install axios

Step 4 — Writing the code

Now, open the processor.js file and paste the code below into it:

// This code requires Node.js. Do not run this code directly in a web browser.

const axios = require("axios");
const FormData = require("form-data");
const fs = require("fs");

const formData = new FormData();
formData.append(
  "instructions",
  JSON.stringify({
    parts: [
      {
        file: "first_half",
      },
      {
        file: "second_half",
      },
    ],
  }),
);
formData.append(
  "first_half",
  fs.createReadStream("input_documents/first_half.pdf"),
);
formData.append(
  "second_half",
  fs.createReadStream("input_documents/second_half.pdf"),
)(async () => {
  try {
    const response = await axios.post(
      "https://api.nutrient.io/build",
      formData,
      {
        headers: formData.getHeaders({
          Authorization: "Bearer YOUR_API_KEY_HERE",
        }),
        responseType: "stream",
      },
    );

    response.data.pipe(
      fs.createWriteStream("processed_documents/node_result.pdf"),
    );
  } catch (e) {
    const errorString = await streamToString(e.response.data);
    console.log(errorString);
  }
})();

function streamToString(stream) {
  const chunks = [];
  return new Promise((resolve, reject) => {
    stream.on("data", (chunk) => chunks.push(Buffer.from(chunk)));
    stream.on("error", (err) => reject(err));
    stream.on("end", () => resolve(Buffer.concat(chunks).toString("utf8")));
  });
}

Code explanation

You first imported all the packages needed to make the API call, and then you prepared the FormData. After that, you appended the first_half and second_half by reading the first_half.pdf and second_half.pdf files from the input_documents folder.

You then made an asynchronous API call to the merge PDF API and stored the result in processed_documents/node_result.pdf.

Step 5 — Output

To run the code, use the command below:

node processor.js

On the successful execution of the code, you’ll see a new processed file named node_result.pdf in the processed_documents folder.

The folder structure will look like this:

merge_pdf
├── input_documents
|    └── first_half.pdf
|    └── second_half.pdf
├── node_modules
├── processed_documents
|    └── node_result.pdf

You can now easily integrate this function into your other JavaScript modules and merge different documents.

Conclusion

In this post, you learned how to merge files for your JavaScript application into a single PDF using our merge PDF API.

If you have a more complex use case, you can use our other tools to add watermarks, perform OCR, and edit (split, flatten, delete, duplicate) documents — and you can even combine these tools. To get started with a free trial, sign up here(opens in a new tab).

Additional resources

Explore more ways to work with Nutrient API:

• Postman collection — Test API endpoints directly in Postman
• Zapier integration — Automate document workflows without code
• MCP Server — PDF automation for LLM applications
• JavaScript client — Official JavaScript library

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