If you do not know how to run complex search queries to find PDF material online, then you should bookmark The Manuals. It is one of the best free PDF Manual search engine.
Actually it is not a search engine, but the owner has written a well customized script to dig PDF’s from Google and put across the result in a very presentable format. This saves your time if you are searching for any products user manual or other PDF’s online. They claim to have more than 57 million PDF files currently !!
They also have a ‘Search Plugin’, which you can add to your Mozilla Firefox browser if you like this search feature.
A definite time saver for those who keep researching and downloading a lot of PDF Manuals online.