Creating a custom shortcut for Google Docs (Alt + P download as pdf)

I wrote the following script to download as PDF a Google Docs file; when I click Alt + P nothing happens. I'm using Google Chrome.

// ==UserScript==
// @name         Google Docs PDF Download Shortcut
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Adds a keyboard shortcut to download Google Docs as PDF
// @author       You
// @match        https://docs.google.com/document/d/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';

    document.addEventListener('keydown', function(e) {
        if (e.altKey && e.key === 'p') {
            let fileId = window.location.pathname.split('/')[3];
            let downloadUrl = `https://docs.google.com/document/d/${fileId}/export?format=pdf`;
            window.open(downloadUrl, '_blank');
        }
    });
})();

What can I do to fix this issue?

It seems that, when the editor is focused, all keydown events are fired in an about:blank iframe. Since Tampermonkey doesn't allow userscripts to run in about:blank, monitoring keydown events on document won't work when the editor is in focus.

Instead, you can wait for the iframe to appear and then add your listener using a modified version of this hack:

// ==UserScript==
// ...
// @match        https://docs.google.com/document/*
// @run-at       document-start
// ==/UserScript==

(function() {
    'use strict';

    function openInNewTab(event) {
        if (event.altKey && event.key === 'p') {
            const fileId = window.location.pathname.split('/')[3];
            const downloadUrl = `https://docs.google.com/document/d/${fileId}/export?format=pdf`;

            window.open(downloadUrl, '_blank');
        }
    }

    document.addEventListener('keydown', openInNewTab);

    setTimeout(function poll() {
        const iframe = document.querySelector('iframe.docs-texteventtarget-iframe');

        if (!iframe) return setTimeout(poll, 100);

        const doc = iframe?.contentDocument;
        const div = doc?.querySelector('div');

        if (!div) return setTimeout(poll, 100);

        div.addEventListener('keydown', openInNewTab);
    });
})();