CSAW 2014 Forensics 200(2) Obscurity

This problem had 635 solves by the end of the competition. It was fairly easy.
It provided you with a file: pdf.pdf . The first thing I did after looking at the file is analyze it with scalpel and HxD to look for fun things inside the file. I didn't see anything, so I took another look at the pdf in Adobe Reader. There was a suspicious amount of whitespace at the top of the file, so I Edit>Select All and saw a suspicious blue box in the middle of the picture. Surprise! There was a key hidden beneath the photo. 'flag{security_through_obscurity}'

Thanks for reading!

-blkbrd