BitDefender's researchers said the number of spam messages with PDF attachments dropped recently. This is because recipients are not clicking on uninvited PDFs, said Vlad Valceanu, head of BitDefender's anti-spam lab.
BitDefender rival MessageLabs said two weeks ago that hackers had tweaked the Storm worm to produce the infected PDF files. These took the form of fake job advertisements, greetings cards, and stock tips to support penny-stock "pump-and-dump" schemes.
Some user companies worried that text-based anti-spam software could not filter such content accurately, and blocking PDF attachments was not an option.
"While spammers are sending out fewer attachments with their e-mails, we do not expect to see a dramatic decrease in the overall amount of spam distributed," said Valceanu. "Our research has simply found that spammers are finding newer, more productive methods in their delivery of spam messages."