Contract PDFs in Next.js

Why contracts become PDFs

A contract has to be fixed at signature. Both parties need to be certain they are looking at identical words, which a rendered page cannot guarantee. That is the difference between a document and a view of a database, and it is why legal and sales operations keep asking for this.

What a contract has to carry

Before any of the code below matters, the template has to produce a document that is actually a contract. These are the fields that make it one, and the ones a reader or an auditor will look for first.

• the parties, by legal entity name and registered number
• the effective date and the term
• numbered clauses with stable numbering, since amendments reference them
• the commercial schedule, usually as an annexe rather than in the body
• signature blocks with name, title and date for each party

Turning your React, rendered to a string on the server template into a document

You already have the markup. RenderToStaticMarkup(<Invoice {...data} />) gives you it as a string, which is the only input the render needs. This is the framework where the incumbent approach fails hardest. A serverless function has a hard bundle size limit, and a Chromium build does not fit inside it. That is why the ecosystem grew a whole genre of stripped Chromium packages, and why an HTTP render is the path of least resistance here rather than an optimisation.

The Next.js implementation

fetch ships with the runtime, so this adds no dependency. The render happens in a Route Handler returning a Response, and you return a Response with the upstream body and a PDF content type.

// app/contracts/[id]/pdf/route.ts
import { renderToStaticMarkup } from "react-dom/server";
import { ContractDocument } from "@/documents/contract";
import { documentStyles } from "@/documents/styles";

export async function GET(
  _request: Request,
  { params }: { params: Promise<{ id: string }> },
) {
  const { id } = await params;
  const contract = await getContract(id);

  // The style tag is the load-bearing part. Your app's CSS lives in a bundle the
  // renderer will never fetch, so the document needs its styles inline.
  const html = `<!doctype html><html><head><meta charset="utf-8">
<style>${documentStyles}</style></head><body>${renderToStaticMarkup(
    <ContractDocument contract={contract} />,
  )}</body></html>`;

  const upstream = await fetch("https://api.pdfpipe.xyz/v1/pdf", {
    method: "POST",
    headers: {
      Authorization: `Bearer ${process.env.PDFPIPE_KEY}`,
      "Content-Type": "application/json",
    },
    body: JSON.stringify({
      html,
      options: { format: "A4", printBackground: true },
    }),
  });

  if (!upstream.ok) {
    return new Response("render failed", { status: 502 });
  }

  return new Response(upstream.body, {
    headers: {
      "Content-Type": "application/pdf",
      "Content-Disposition": `attachment; filename="contract-${id}.pdf"`,
    },
  });
}

The CSS that makes a contract page correctly

The layout problem specific to this document is that numbered clauses with cross-references, where a page break in the wrong place separates a clause from the term it defines. These rules handle it.

/* A clause must not be separated from the term it defines, and a
   signature block alone on a final page invites a dispute. */
.clause     { break-inside: avoid; }
.signatures { break-inside: avoid; break-before: avoid; }

p { orphans: 3; widows: 3; }

@page { size: A4; margin: 25mm 20mm; }

What goes wrong in Next.js

renderToStaticMarkup gives you markup with no stylesheet attached, because your CSS lives in a bundle the PDF renderer will never load. Inline the styles into a style tag in the string you send, or the document renders with browser defaults.

What people try first

Most Next.js projects reach for puppeteer-core paired with a size-reduced Chromium package, which works but ties your deployment to a binary that has to be rebuilt for every runtime version bump. That works until it is running on more than one machine, at which point the browser becomes the thing you operate rather than the thing you use.

Where the contract lives afterwards

Rendering is the short part. A contract is retained for the term plus the limitation period, which typically means well over a decade. Immutability matters more here than anywhere else on this list: both parties must be able to prove the words have not moved.

Getting the document right

• Check the parties and the effective date, which is what every later dispute turns on.
• Generation is triggered by agreement being reached, often with a signature workflow following, so size the timeout for that path rather than for a health check.
• These are produced one at a time by a person who is waiting, so latency is felt directly.
• Backgrounds are painted by default here, so a design that uses colour needs nothing set. Only an explicit print_background of false turns them off.

Frequently asked

Related

Other Next.js documents, and the same contract in other stacks.