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">
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…