Security experts have reported a "dramatic decrease" in the amount of spam emails using PDF file attachments to spread unsolicited messages.

Research compiled by SophosLabs reveals that levels of PDF spam have dropped from a high of nearly 30 per cent of all spam earlier this month, to virtually zero.

"If PDF spam email messages have all but disappeared, there can only be one reason: they are not working," said Graham Cluley, senior technology consultant at Sophos.

"Spammers would not turn away from PDF spam if it was an effective way to fill their pockets with cash and direct consumers to their websites, dodgy goods or dodgy investment opportunities.

"This 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 per cent increase in overall spam levels. Since then PDF spam has shown a sharp decline.

"Of course, it is too early to say that this is the last we will see of PDF spam. There could still be more campaigns to come, but its dramatic fall may be a sign that we are witnessing its demise," added Cluley.