PDFfetch.com will, unsurprisingly, find you PDF files.
They say of themselves "No matter you are looking for .pdf files, ebooks, digital publications, presentations or electronic documents - with an exclusive technology, PDFfetch gives you access to a huge library of documents instantaneously." It's very simple - you run your basic search in their search box, look at the results in title, URL and summary form and off you go. It's created with Yahoo! Search, and I couldn't get a real indication as to how many results it returned, so it's not easy to compare it to other engines that offer a filetype: search function. However, I did manage to do a very specific search that returned 7 results for me, but when I repeated the search on Google I got over 900. So I do have to wonder slightly at just how comprehensive it is!