tangor - 8:47 am on Dec 12, 2011 (gmt 0)
BDW, you've left us with little to work with. Site is indexed. No obvious errors in delivery. No possible duplicate content. BTW, a downloadable PDF, unless walled off, can be indexed by the major SEs and thus "known".
I repeat: Could it quite possibly be there is no interest in the content?
Believe me, I've been there, done that!