Recommended way to embed PDF in HTML?

Probably the best approach is to use the PDF.JS library. It's a pure HTML5/JavaScript renderer for PDF documents without any third-party plugins.

Online demo: https://mozilla.github.io/pdf.js/web/viewer.html

GitHub: https://github.com/mozilla/pdf.js

You can also use Google PDF viewer for this purpose. As far as I know it's not an official Google feature (am I wrong on this?), but it works for me very nicely and smoothly. You need to upload your PDF somewhere before and just use its URL:

<iframe src="http://docs.google.com/gview?url=http://example.com/mypdf.pdf&embedded=true" style="width:718px; height:700px;" frameborder="0"></iframe>

What is important is that it doesn't need a Flash player, it uses JavaScript.

This is quick, easy, to the point and doesn't require any third-party script:

<embed src="http://example.com/the.pdf" width="500" height="375" 
 type="application/pdf">

UPDATE (2/3/2021)

Adobe now offers it's own PDF Embed API.

https://www.adobe.io/apis/documentcloud/dcsdk/pdf-embed.html

UPDATE (1/2018):

The Chrome browser on Android no longer supports PDF embeds. You can get around this by using the Google Drive PDF viewer

<embed src="https://drive.google.com/viewerng/
viewer?embedded=true&url=http://example.com/the.pdf" width="500" height="375">