Introduction

Using async and await for PDF generation usually means wrapping the file-writing or stream-completion phase in a promise. The important distinction is that async and await do not magically make CPU work disappear; they simply give you a clean way to wait for asynchronous I/O such as writing the generated PDF to disk or sending it in an HTTP response.

What Is Actually Asynchronous

Many PDF libraries perform document-building code synchronously while the output stream finishes asynchronously. That means your code may still construct the PDF content immediately, but the final write to a file or network stream completes later.

This is why an async wrapper is often needed. You want to wait until the stream has actually finished before continuing.

Wrapping PDF Generation in a Promise

A practical Node.js example with pdfkit is to return a promise that resolves when the writable stream emits finish.

1const fs = require("fs");
2const PDFDocument = require("pdfkit");
3
4function generatePdf(filePath, text) {
5  return new Promise((resolve, reject) => {
6    const doc = new PDFDocument();
7    const stream = fs.createWriteStream(filePath);
8
9    stream.on("finish", resolve);
10    stream.on("error", reject);
11    doc.on("error", reject);
12
13    doc.pipe(stream);
14    doc.fontSize(18).text(text);
15    doc.end();
16  });
17}

Now the caller can use await cleanly:

1async function main() {
2  await generatePdf("./report.pdf", "Quarterly report");
3  console.log("PDF written");
4}
5
6main().catch(console.error);

This is the usual pattern: the promise resolves when the output is actually complete.

Using It in an Express Route

In a web application, you often want to wait for PDF generation before responding or before moving to the next async step.

1const express = require("express");
2
3const app = express();
4
5app.get("/report", async (req, res, next) => {
6  try {
7    await generatePdf("./report.pdf", "Generated from Express");
8    res.send("PDF generated");
9  } catch (err) {
10    next(err);
11  }
12});

In a real system, you might stream the PDF directly to the response instead of writing it to disk first, but the async principle is the same: await the operation that actually finishes the output.

Do Not Assume async Makes Heavy Work Non-Blocking

If the PDF library performs expensive CPU work in JavaScript before the stream phase, async and await do not move that work off the event loop automatically. They help you express asynchronous flow, but they do not turn CPU-bound logic into background computation.

That means there are two separate concerns:

• promise-based control flow,
• event-loop blocking caused by heavy rendering.

If PDF generation becomes expensive, you may need a worker process, a job queue, or a separate service rather than only rewriting the function with async and await.

Return the Promise, Do Not Hide It

One common mistake is starting PDF generation and not returning the promise to the caller. That makes the outer async function appear complete too early.

Bad pattern:

1async function buildReport() {
2  generatePdf("./report.pdf", "Hello");
3  console.log("Finished");
4}

This logs "Finished" before the stream necessarily ends. The correct version awaits or returns the promise:

1async function buildReport() {
2  await generatePdf("./report.pdf", "Hello");
3  console.log("Finished");
4}

That one change is often the difference between reliable sequencing and race-condition bugs.

Common Pitfalls

• Assuming async alone makes PDF creation non-blocking even when the library does heavy CPU work synchronously.
• Resolving the promise too early instead of waiting for the stream to finish.
• Forgetting to handle stream errors, which leaves the promise hanging or the request failing unclearly.
• Starting PDF generation inside an async function without returning or awaiting the promise.
• Writing temporary files to disk when direct response streaming would fit the use case better.

Summary

• 'async and await are useful for sequencing PDF generation around asynchronous stream completion.'
• The usual pattern is to wrap the writable stream in a promise and resolve it on finish.
• Await the promise so the caller knows when the PDF is actually ready.
• Remember that async does not eliminate CPU-bound event-loop blocking by itself.
• If generation becomes heavy, consider a worker or background job architecture instead of only changing syntax.