PDF spam no more?

Over approximately the past 2 months, PDF spam has exploded from a little used technique to making up close to 30% of all spam being sent during its peak (averaged daily). Due to spammers adjusting their campaigns, the volume of these messages has fluctuated, however over the past week PDF spam has all but dried up.

The following is a graph illustrating what the percentage of all spam received on SophosLabs traps contain an attached PDF:

PDF spam hit rate

Since August 21st, the volume has dropped off the graph. Could this mean the spammers behind these campaigns have moved on to develop other techniques? Taking a holiday? Only time will tell.