PDF spam is fairly easy to filter out... We have been automatically blocking it now for a couple of weeks (At www.ClearMyMail.com). Over the last few months the spammers have become a lot more professional in their methods.
They are testing and monitoring various new techniques, the PDF spam is just one of the many ways they are trying to get ahead of the spam filters. They are using methods that direct mailers have used for years... testing and evolving the methods that get the best response.
They are currently testing office type attachments. Excel & word files that claim to be invoices for example. These types of attachments cannot be blocked by type and need further investigation by the spam filter to determine what the content is.
Its a constant battle between the spam filters and the anti-spam companies, one that we aim to keep in the lead of!