How to use wget to download certain files from a certain directory when the directory itself does not have an index.html?

There are a number of questions similar to this on StackExchange but none address this issue.

More specifically I want to download all the pdf files in the 2007 directory at http://ift.tt/157UtPd.

So I want wget to parse the html file available at the above link and only follow links that go to pdf files in the 2007 directory.

I used the following but it didn't work:

wget -r -A pdf -I /2007 'http://ift.tt/157UtPd'

Can you also explain why the above does not work?