Flatten pdf with poppler utils pdftocairo

index.ts

import { EventEmitter } from "events";
import { spawn, ChildProcess } from "child_process";
import { Writable } from "stream";
import fs from "fs";

/**
 * from: https://github.com/rauschma/stringio/
 */
function streamPromiseHelper(
  emitter: EventEmitter,
  operation: (callback: () => void) => void
): Promise<void> {
  return new Promise((resolve, reject) => {
    const errListener = (err: Error) => {
      emitter.removeListener("error", errListener);
      reject(err);
    };
    emitter.addListener("error", errListener);
    const callback = () => {
      emitter.removeListener("error", errListener);
      resolve(undefined);
    };
    operation(callback);
  });
}

/**
 * from: https://github.com/rauschma/stringio/
 */
function streamWrite(
  stream: Writable,
  chunk: string | Buffer | Uint8Array,
  encoding = "utf8"
): Promise<void> {
  // Get notified via callback when it’s “safe” to write again.
  // The alternatives are:
  // – 'drain' event waits until buffering is below “high water mark”
  // – callback waits until written content is unbuffered
  return streamPromiseHelper(stream, callback =>
    stream.write(chunk, encoding, callback)
  );
}

/**
 * from: https://github.com/rauschma/stringio/
 */
function streamEnd(stream: Writable): Promise<void> {
  return streamPromiseHelper(stream, callback => stream.end(callback));
}

async function streamStdOutToBuffer(
  childProcess: ChildProcess
): Promise<Buffer> {
  return new Promise((resolve, reject) => {
    let buffer = Buffer.from("");

    childProcess.stdout.on("data", data => {
      buffer = Buffer.concat([buffer, data]);
    });

    childProcess.once("exit", (code, signal) => {
      if (code === 0) {
        resolve(buffer);
      } else {
        reject(new Error("Exit with error code: " + code));
      }
    });

    childProcess.once("error", error => {
      reject(error);
    });
  });
}

async function flattenPdf(pdf: Buffer): Promise<Buffer> {
  const childProcess = spawn("pdftocairo", ["-pdf", "-", "-"]);

  await streamWrite(childProcess.stdin, pdf);
  await streamEnd(childProcess.stdin);

  return await streamStdOutToBuffer(childProcess);
}

/**
 * Example
 */
const main = async () => {
  const pdf = fs.readFileSync("./invoice-gb.pdf");
  const flattenedPdf = await flattenPdf(pdf);
  fs.writeFileSync("./foobar.pdf", flattenedPdf);
};

main();

Thanks for attaching this.