While there is way to configure Google Search to specifically search for PDF (e-books), here is similar alternative for the same. PDF Search Engine allows you to search web for e-books with relative ease.
Just enter the keyword for search and hit the ’search’ button. It displays the related list of PDF e-books ready for download. Right click on any result and then click on ‘Save as target’ to download an e-book.
PDF Search Engine website also display list of related tags at the bottom of the webpage. This Google powered PDF search is one handy way to search PDF Files on the fly.