Google Chrome browser's built-in PDF plugin uses Foxit PDF SDK; "With incredible small size (less than 2MB) and amazingly fast speed, Foxit PDF SDK (DLL) 2.0 is your absolute choice to develop your own PDF apps," suggests Foxit.
Google decided to include two of the most popular plugins (Flash and PDF) in Google Chrome to make the browser more secure. "PDF plugin runs inside Chrome's sandbox so that vulnerabilities can no longer install malware on a computer. And, Flash is automatically updated by Google and it's likely to run inside Chrome's sandbox in the future. Google Chrome 5 already includes Flash plugin, while PDF plugin is available in Google Chrome 6 (dev, beta)."