magento-test/app/code/Magento/Downloadable/etc/pdf.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Sales:etc/pdf_file.xsd">
    <renderers>
        <page type="invoice">
            <renderer product_type="downloadable">Magento\Downloadable\Model\Sales\Order\Pdf\Items\Invoice</renderer>
        </page>
        <page type="creditmemo">
            <renderer product_type="downloadable">Magento\Downloadable\Model\Sales\Order\Pdf\Items\Creditmemo</renderer>
        </page>
    </renderers>
</config>