How to flatten a PDF using JavaScript

This post shows how to flatten PDFs in JavaScript using the flatten PDF JavaScript API. The free plan includes 200 credits. Different operations consume different amounts of credits, so the number of PDFs you can process varies. Create a free account(opens in a new tab) to get your API key.

Why flatten PDFs?

Flattening PDFs is essential for document workflows that require finalized, non-editable outputs. Common use cases include:

• Print preparation — Merge all layers — including annotations, form fields, and signatures — into a single layer to ensure printers render the complete document correctly.
• Secure distribution — Lock completed forms to prevent recipients from modifying submitted data.
• Archival compliance — Create immutable document versions for legal and regulatory requirements.
• Consistent rendering — Guarantee documents appear identical across all systems and PDF readers.
• Workflow automation — Process batches of documents programmatically as part of your backend infrastructure.

Nutrient DWS Processor API

Document flattening is one of 30+ PDF API tools. You can combine flattening with other tools to build document processing workflows:

• Convert MS Office files and images to PDF, and then flatten
• Merge multiple documents, and then flatten the result
• Add watermarks and signatures, and then flatten

After creating your account, you can access all PDF API tools.

Step 1 — Creating a free account on Nutrient

Go to the signup page(opens in a new tab) to create your free account.

After creating your account, you’ll see your plan overview.

The bottom-left corner shows your 200 starting credits and access to all PDF API tools.

Step 2 — Obtaining the API key

After verifying your email, get your API key from the dashboard. Click API keys in the left menu to see your keys overview.

Copy the Live API key for the flatten PDF API.

Step 3 — Setting up folders and files

Create a folder called flatten_pdf and open it in your code editor. Create two subfolders: input_documents and processed_documents. Copy your PDF file to input_documents and rename it to document.pdf.

Create a file called processor.js in the root folder for your code.

Your folder structure:

flatten_pdf
├── input_documents
|    └── document.pdf
├── processed_documents
└── processor.js

Step 4 — Installing dependencies

Install these dependencies:

• axios(opens in a new tab) — Makes REST API calls
• Form-Data(opens in a new tab) — Creates form data

Install both:

npm install axios
npm install form-data

Step 5 — Writing the code

Add this code to processor.js:

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

const formData = new FormData();
formData.append(
  'instructions',
  JSON.stringify({
    parts: [
      {
        file: 'document',
      },
    ],
    actions: [
      {
        type: 'flatten',
      },
    ],
  }),
);
formData.append(
  'document',
  fs.createReadStream('input_documents/document.pdf'),
);
(async () => {
  try {
    const response = await axios.post(
      'https://api.pspdfkit.com/build',
      formData,
      {
        headers: formData.getHeaders({
          Authorization: 'Bearer YOUR_API_KEY_HERE',
        }),
        responseType: 'stream',
      },
    );

    response.data.pipe(
      fs.createWriteStream('processed_documents/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

The FormData variable contains the API instructions, and createReadStream reads the input PDF file. The POST request sends data to the flatten PDF API, and the API response saves to the processed_documents folder.

Output

Run the code:

node processor.js

After successful execution, result.pdf appears in the processed_documents folder.

Final folder structure:

flatten_pdf
├── input_documents
|    └── document.pdf
├── processed_documents
|    └── result.pdf
└── processor.js

Conclusion

You now know how to flatten PDFs in JavaScript using the flatten PDF API.

Integrate these functions into your applications to flatten PDFs. The same API token works for other operations like merging documents and adding watermarks. Sign up(opens in a new tab) for a free trial.

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