1. Scribd (already mentioned at SEJ) is a huge user-generated database of eBooks and community of writers. Due to the huge volume of eBooks it contains and also being user-generated (and thus used for self-promotion most of the time) in nature, it is not so easy to find something valuable there. You can try to browse its groups. There are several really cool groups there found for [SEO] search each containing really good books.
2. Google offers a nifty advanced operator allowing to search for any file types: {filetype:} or {ext:} – that’s my preferred way to search for eBooks and pdf reports (as well as search for anything actually). First, I am just used to Google search and secondly, Google search works the best for me: it returns most relevant results, can be customized and restricted with other advanced operators.
The only disadvantage of it is that it returns only one or two results from one domain (and thus you risk missing plenty of useful and important documents stored at popular eBook directories like Scribd).
3. PDFdatabase claims to contain more than 7 million pdf and doc files. The search is quite fast and focused. The search results are pretty self-explanatory: you will see the document name, size, the full URL, the related search tags and the download link:
(Tip credit to Addictive Tips)
Other similar PDF search engines you can check out: PDF search engine and PDFgeni.
