I had a "quick" look at the site in question, and the pdf always seems to get oaded from the same link, which "becomes" a link to the selected pdf .. so I'm guessing it's ll scripted.
Nor wll it let you "browse" the directory it says the pdf comes from.
That said, playing with wget::gui and trying a few random options seemed to download quite a few directories .. but no pdf's, just a bunch of xml files, even though I'd told it to get pdf's
(maybe I just didn't give it enough time to follow all the links).
I'd also guess there will be gigabytes of pdf's on that site .. which would explain them not wanting 1000's of people all downloading the lot in one go .. they'd lock up the site for hours.