We’ve been seeing a gradual shift in malicious PDF file coding (no surprise there, we know malware authors can and do adapt their techniques). For a long time, we saw malicious PDF files that were simple enough to allow us to readily decipher the intent of the malicious code — shell code, download/execute, drop and load, et cetera. Now we’re seeing more and more complex obfuscation being used, which requires us to break down the PDF file.

Excerpt from:
Analyzing PDF Files