Symfony: Streamed response of external command to the browser to display real time progress of the command

I'm generating a large PDF with 2000 pages in symfony (4.2) framework. What I'm doing is just save the HTML content to the .HTML file by getting content from the twig.

Then I'm using the headless chrome to generate the PDF from the URL using the below command.

/usr/bin/google-chrome --headless --disable-gpu --run-all-compositor-stages-before-draw --print-to-pdf [URL of HTML file] --virtual-time-budget=10000

Now, the requirement is while the above command is running I have to display the loader with the progress bar in the front.

What I did is as below to get the stream response and display them on the browser.

Controller

public function streamAction()
{
    $process = new Process(["pwd"]);
    $process->run();

    $output = new StreamedOutputService(fopen('php://stdout', 'w'));

    $response = new StreamedResponse(function() use ($output, $process) {
        // $process->isRunning() always returns false.
        while ($process->isRunning()) {
            $output->writeln($process->getOutput());
        }
    });
    $response->headers->set('X-Accel-Buffering', 'no');

    return $response;
}

Streamed Response Class

protected function doWrite($message, $newline)
{
    if (
        false === @fwrite($this->getStream(), $message) ||
        (
            $newline &&
            (false === @fwrite($this->getStream(), PHP_EOL))
        )
    ) {
        throw new RuntimeException('Unable to write output.');
    }

    echo $message;

    ob_flush();
    flush();
}

What is the buggy on the above code? I'm not able to get the output of the command hence can not write it to the browser.

Below code is working fine and sending response at every 2 seconds on the browser

public function streamAction()
{
    $output = new StreamedOutputService(fopen('php://stdout', 'w'));

    $response = new StreamedResponse(function() use ($output) {
        for($i = 0; $i <= 5; $i++) {
             $output->writeln($i);
             sleep(2);
        }
    });
    $response->headers->set('X-Accel-Buffering', 'no');

    return $response;
}

Thanks in advance.