There has been a dramatic decrease in the amount of spam emails using PDF file attachments to spread their unwanted messages.  According to research compiled by SophosLabs, levels of PDF spam have dropped from a high of close to 30% of all spam earlier this month, to almost zero. 

If PDF spam email messages have all but disappeared, there can only be one reason – they're not working.  
"Spammers wouldn't turn away from PDF spam if it was an effective way to make money and direct consumers to their websites, dodgy goods or investment opportunities,” says Brett Myroff, CEO of master Sophos distributor, NetXactics.  The drop indicates that the spammers are finding it hard to fool the public into reading marketing messages distributed in this way.
Levels of PDF spam spiked on 7 August 2007 when a single campaign, designed to manipulate stock prices of Prime Time Group Inc, accounted for a 30% increase in overall junk email levels.  Since then, however, PDF spam has shown a sharp decline.
"It is, however, too early to say whether this is the last we will see of PDF spam.  While there could be more campaigns to come, its dramatic fall may indicate its demise," says Myroff.
Sophos’s advice remains the same to all internet users in that it makes sense to ensure that email inboxes are protected with a product that can defend against spam and malware.
Sophos experts point to a number of disadvantages for spammers who try and use PDFs in their spam campaigns which may explain its decline.
"PDF spam isn't as immediate as an instant glimpse of the marketing message in your email preview pane," explains Myroff. "Adobe Acrobat also takes a fair amount of time to load before the contents of the PDF are revealed. Consumers quickly learn that it's a waste of time to open every unsolicited PDF they receive, which means the spammer's message doesn't get read, and cybercriminals don't make any money."