Web bugs are little graphic “bugs” that are usually used in email. Some newsletter publishers use these to track who opens up their newsletters. Spammers also use them to see if they have a “live body”.
Web bugs get activated if you view an email, even in Outlook’s preview pane. That’s why Outlook 2003 has the ability to not download graphics (which is why newsletters and such can come through looking all broken-up).
According to Remote Approach, opening up a PDF file enabled with Remote Approach gives the following information: IP Address, domain type (com, edu, gov, etc.) and other stuff like what kind of browser you use, your local time, what service provider you use, etc.
Note that this type of data collection is not unusual on the web (every time you got a website, the website operator can collect this type of data). It’s not getting your name, address, credit card number, etc. However, it’s the first I’ve heard of it for PDF documents.