PDF flood
         

Stock spammers pumping Prime Time Group Inc have released a flood of PDF spam, which some security vendors claim as the largest ever such flood. It's certainly intense — we're seeing hundreds of messages a day, spread across a wide range of addresses. Addresses targeted include a large number of 'non-existent' addresses, including addresses that were created by other spammers for use in 'From:' lines and have now been picked up and are getting spam in their own right.
         

http://www.spamnation.info/blog/archives/2007/08/pdf_flood.html