There are a couple of reasons to believe that this is not the case.
First, there were thousands of them. Someone having thousands of books is not unreasonable, of course, but both the breadth and depth of this collection is such that it is extremely unlikely it is someone's personal library.
Second, the PDFs aren't the actual books. They are just short blurbs describing the book and containing download deep links into bookfreenow.com. A couple examples [1] [2]. Clicking to create an account so you can start downloading redirects through some ad companies (and possibly some shady affiliate marketing companies), eventually reaching some download site (I think) that tells you no free slots are available and asks you to make an account.
(The bookfreenow.com pages for each book all seem to be the same template with just the book info substituted. Even the comments on the each page are from the same people, at the same times, and say the exact same things except they have the correct book title on each page. They aren't even trying to make it look like the comments are legit).
[1] http://bookfreenow.com/downloads/the-lm3900-a-new-current-di...
[2] http://bookfreenow.com/downloads/essential-orthopaedics-by-j...