I think you will find that unless you fork/rewrite lightbox2 it is intentionally limited to images, which it does so well there are many clones including paid for ones that do very little extra (i.e. also limited to static images)
However the Fancybox you mention has various versions and version 3 looks well polished with many options.

But by far the simplest is Iframe-Box the options are less but that can be a bonus for simplicity.

<!DOCTYPE html><html lang="en-us">
<head>
<meta charset="utf-8">
<title>IFrame-box Example</title>
<script type="text/javascript" src="https://code.jquery.com/jquery-1.7.2.js"></script>
<link rel="stylesheet" href="./css/framer.css">
</head>
<body><p style="color :red; ">Introduction, click to
<a href="http://africau.edu/images/default/sample.pdf" data-framer="iframe-1" data-title="A descriptive caption or instruction<br>
Can include inline break for second caption line<br> Can include inline break for third caption line<br>Etc.">open a PDF</a>
Any other page Text </p>
<script src="./js/framer.js"></script></body></html>