A new type of spam has been finding its way into inboxes by taking advantage of a well-worn technology: PDF files.

In recent weeks, spammers have been bypassing corporate email filters by hiding their content in PDF attachments instead of the body of messages. Like most junk email, the messages hawk things like penny stocks, prescription drugs and lotteries.

The so-called PDF spam is the latest volley in the cat-and-mouse game between spam senders and network defenders. Despite the federal Can-Spam Act of 2003 and heavy spending by corporations on antispam technologies, junk email remains a problem. It accounts for more than three-quarters of email transmitted over public networks, according to Ferris Research Inc., a San Francisco market-research firm. Spammer techniques have become increasingly sophisticated as filtering methods have improved, and spam fighters admit that they frequently play a reactive role.

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