Spammers experimenting with attached PDF, Excel, Text and ZIP files has led to a massive spike in the amount junk mail arriving in people's inboxes.

Such emails now represent almost 25 per cent of all spam, up from only two per cent last week, according to email and internet content security provider Marshal.

The company said this type of junk email is rapidly replacing the previously dominant image-based spam.

Spammers use it to try to bypass spam filters, and even Web savvy users are more likely to open such emails, because attachments such as PDF files have a reputation for being safe.

"What we are seeing is the spammer's equivalent to a comparison study. They want to see which types of attachments they have the most success with and which are the most effective.

"The spammers' experimentation with a range of attachments indicates they are not confident PDF spam is as effective as image spam was in the past," said Bradley Anstis, Marshal's product management director.

Most of this spam is Stock Spam with PDF the favoured format, closely followed by a trend for other attachments such as Excel files, Text and ZIP files.

Marshal said its research from its Threat Research and Content Engineering (TRACE) centre indicates that one core group of spammers is responsible for the significant majority of Attachment Spam.

"All the signs are that it is the same group of spammers behind the Storm worm," said Anstis.