Here is my solution, please check this out:

var myWindow = window.open('<myfile>.pdf','my window','resizable,scrollbars');
myWindow.document.write('<title>My PDF File Title</title>');

Hope I could help.

Here is my solution, please check this out:

var myWindow = window.open('<myfile>.pdf','my window','resizable,scrollbars');
myWindow.document.write('<title>My PDF File Title</title>');

Hope I could help.

I was having problems with the accepted answer until I realized that if you open an existing, slow page that already has a <title> the browser will 1) set your title, then 2) once the document fully loads it will (re)set the popup title with the "normal" value.

So, introducing a reasonable delay (function openPopupWithTitle):

var overridePopupTitle = function(popup, title, delayFinal, delayRepeat) {
    // https://stackoverflow.com/a/7501545/1037948
    // delay writing the title until after it's fully loaded,
    // because the webpage's actual title may take some time to appear
    if(popup.document) setTimeout(function() { popup.document.title = title; }, delayFinal || 1000);
    else setTimeout(function() { overridePopupTitle(popup, title); }, delayRepeat || 100);
}
var openPopupWithTitle = function(url, title, settings, delay) {
    var win = window.open(url, title, settings);
    overridePopupTitle(win, title, delay);
    return win;
}

This worked for me.

var newWindow = window.open(newdata, "_blank");
setTimeout(function(){ newWindow.document.title = 'my new title'; }, 1000);