Subject: IMPORTANT: Royal Mail Delivery Invoice #1092817
From: "Royal Mail" <firstname.lastname@example.org>
Date: Wed, April 14, 2010 11:28 am
We missed you, when trying to deliver.
Please view the invoice and contact us with any questions.
We will try to deliver again the following business day.
The bad PDF file looks like some sort of calendar, I have not yet been able to analyse exactly what sort of evil things it does.
If you still use Adobe Acrobat then you should make sure that you update to the latest version which is 9.3.2, or use an alternative like Sumatra.