Since after the pdf file is loaded, the iframe document will have a new DOM element <embed/>, so we can do the check like this:
window.onload = function () {
//creating an iframe element
var ifr = document.createElement('iframe');
document.body.appendChild(ifr);
// making the iframe fill the viewport
ifr.width = '100%';
ifr.height = window.innerHeight;
// continuously checking to see if the pdf file has been loaded
self.interval = setInterval(function () {
if (ifr && ifr.contentDocument && ifr.contentDocument.readyState === 'complete' && ifr.contentDocument.embeds && ifr.contentDocument.embeds.length > 0) {
clearInterval(self.interval);
console.log("loaded");
//You can do print here: ifr.contentWindow.print();
}
}, 100);
ifr.src = src;
}