My favorite ebook search engine is Inkmesh. This site helps me find free and paid ebooks in any of the major ebookstores, but when it comes to PDFs Inkmesh also has a couple defects. It only searches ebookstores and it doesn't sort by format.
The thing about PDFs is that there's a lot of noncommercial content released as a PDF which can't be found in another format. Consider your average user manual, owner's manual, or tech support docs. Inkmesh won't see these files at all because they're not for sale anywhere, and Google might not be able to find them in all the vast amounts of content cluttering the web. But PDF SB will find these ebooks, or at least it has a better shot at it.
Every so often I get requests for help in finding info on a device. This search engine should make it a lot easier to find the manuals, some of which are rather well hidden on the various support sites. It currently claims to list over 40 million files. It was originally launched just over 5 months ago, and recently went through a series of updates where the servers were updated and the algorithms were refined.
Positive hype aside, I'll be honest and say that I'm not sure this search engine does a better job than Google. I tested for several PDFs which I know can be found through Google; PDF SB couldn't find them. Mike cane has also pointed out that this search engine ignores some basic tricks like quotation marks.
Still, I'm hoping the site will improve, and at the very least I plan to bookmark it.