How to download a file on Next.js using an API route

I'm using next.js. I have a 3rd party service I need to retrieve a PDF file from. The service requires an API key that I don't want exposed on the client side.

Here are my files

/api/getPDFFile.js ...

  const options = {
    method: 'GET',
    encoding: 'binary',
    headers: {
      'Subscription-Key': process.env.GUIDE_STAR_CHARITY_CHECK_API_PDF_KEY,
      'Content-Type': 'application/json',
    },
    rejectUnauthorized: false,
  };

  const binaryStream = await fetch(
    'https://apidata.guidestar.org/charitycheckpdf/v1/pdf/26-4775012',
    options
  );
  
  return res.status(200).send({body: { data: binaryStream}}); 

pages/getPDF.js

   <button type="button" onClick={() => {
  fetch('http://localhost:3000/api/guidestar/charitycheckpdf',
    {
      method: 'GET',
      encoding: 'binary',
      responseType: 'blob',
    }).then(response => {
      if (response.status !== 200) {
        throw new Error('Sorry, I could not find that file.');
      }
      return response.blob();
    }).then(blob => {
      const url = window.URL.createObjectURL(blob);
      const a = document.createElement('a');
      a.style.display = 'none';
      a.href = url;
      a.setAttribute('download', 'test.pdf');
      document.body.appendChild(a);
      a.click();
      window.URL.revokeObjectURL(url);
    })}}>Click to Download</button>

Clicking the button downloads a file, but when I open it I see the error message, "Failed to load PDF document."

You appear to be using node-fetch. So, you can do something like this:

// /pages/api/getAPI.js

import stream from 'stream';
import { promisify } from 'util';
import fetch from 'node-fetch';

const pipeline = promisify(stream.pipeline);
const url = 'https://w3.org/WAI/ER/tests/xhtml/testfiles/resources/pdf/dummy.pdf';

const handler = async (req, res) => {
  const response = await fetch(url); // replace this with your API call & options
  if (!response.ok) throw new Error(`unexpected response ${response.statusText}`);

  res.setHeader('Content-Type', 'application/pdf');
  res.setHeader('Content-Disposition', 'attachment; filename=dummy.pdf');
  await pipeline(response.body, res);
};

export default handler;

Then from client:

// /pages/index.js

const IndexPage = () => <a href="/api/getPDF">Download PDF</a>;
export default IndexPage;

CodeSandbox Link (open the deployed URL in a new tab to see it work)

References:

• API Routes | Next.js
• Streams | node-fetch
• How to send a pdf file from Node/Express app to the browser
• <a>: The Anchor element | MDN

PS: I don't think much error handling is necessary in this case. If you wish to be more informative to your user you can. But this much code will also work just fine. In case of error the file download will fail showing "Server Error". Also, I don't see a need to create a blob URL first. You can directly download it in your app as the API is on the same origin.

Earlier I had used request, also posting it here in case someone needs it:

import request from 'request';
const url = 'https://w3.org/WAI/ER/tests/xhtml/testfiles/resources/pdf/dummy.pdf';
export default (_, res) => { request.get(url).pipe(res); };