The SonicWALL database has shown that the amount of spam, phishing and virus attacks continues to grow steadily.

SonicWALL, a software security company based in Sunnvale, California, can access 1.3 million email accounts to track the increase in malicious software.

In particular, there has been an increase in Excel, PDF and ZIP spam, according to the company.

Andrew Klein, senior product manager for SonicWALL's Email Security division, said: "PDF spam demonstrates the continued innovation in spam techniques that attempt to bypass anti-spam detection and trick employees into opening e-mails that give the appearance of legitimate business letters."

The reason that PDF spam is more effective is because the PDF format is widely regarded as an established way for enterprises to communicate data, added Mr Klein.

Trust is a major factor in negating people's "psychological defenses", he continued.

Excel and ZIP spam are successful because of the same reason, SonicWALL warned.

Meanwhile, a pump-and-dump spam campaign to manipulate the stock of Prime Time, a Florida-based company, is continuing, reports PC World.