magento2/app/code/Magento/Sales/Controller/Adminhtml/Invoice/AbstractInvoice/PrintAction.php

<?php
/**
 *
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\Sales\Controller\Adminhtml\Invoice\AbstractInvoice;

use Magento\Framework\App\ResponseInterface;
use Magento\Framework\App\Filesystem\DirectoryList;

abstract class PrintAction extends \Magento\Backend\App\Action
{
    /**
     * Authorization level of a basic admin session
     *
     * @see _isAllowed()
     */
    const ADMIN_RESOURCE = 'Magento_Sales::sales_invoice';

    /**
     * @var \Magento\Framework\App\Response\Http\FileFactory
     */
    protected $_fileFactory;

    /**
     * @var \Magento\Backend\Model\View\Result\ForwardFactory
     */
    protected $resultForwardFactory;

    /**
     * @param \Magento\Backend\App\Action\Context $context
     * @param \Magento\Framework\App\Response\Http\FileFactory $fileFactory
     * @param \Magento\Backend\Model\View\Result\ForwardFactory $resultForwardFactory
     */
    public function __construct(
        \Magento\Backend\App\Action\Context $context,
        \Magento\Framework\App\Response\Http\FileFactory $fileFactory,
        \Magento\Backend\Model\View\Result\ForwardFactory $resultForwardFactory
    ) {
        $this->_fileFactory = $fileFactory;
        parent::__construct($context);
        $this->resultForwardFactory = $resultForwardFactory;
    }

    /**
     * @return ResponseInterface|void
     * @throws \Exception
     */
    public function execute()
    {
        $invoiceId = $this->getRequest()->getParam('invoice_id');
        if ($invoiceId) {
            $invoice = $this->_objectManager->create(
                \Magento\Sales\Api\InvoiceRepositoryInterface::class
            )->get($invoiceId);
            if ($invoice) {
                $pdf = $this->_objectManager->create(\Magento\Sales\Model\Order\Pdf\Invoice::class)->getPdf([$invoice]);
                $date = $this->_objectManager->get(
                    \Magento\Framework\Stdlib\DateTime\DateTime::class
                )->date('Y-m-d_H-i-s');
                $fileContent = ['type' => 'string', 'value' => $pdf->render(), 'rm' => true];

                return $this->_fileFactory->create(
                    'invoice' . $date . '.pdf',
                    $fileContent,
                    DirectoryList::VAR_DIR,
                    'application/pdf'
                );
            }
        } else {
            return $this->resultForwardFactory->create()->forward('noroute');
        }
    }
}