From the Sydney Morning Herald:
A company called Remote Approach is using a feature in Adobe’s PDF Reader to track how many times a PDF document has been accessed, according to a report in the Linux Weekly News.
For the feature to be used, a PDF had to be created, uploaded to the company’s server to be tagged, and then distributed, the report said.
The tracking failed when the PDF was viewed by other readers such as Xpdf and Kpdf, and version 5 of Adobe Reader.
However, when the document was opened using Adobe Reader 7, Remote Approach began logging the number of times it was viewed from the IP address of the computer on which it was opened.
The information is submitted over port 80 using HTTP – the standard port for web servers – which no home or office firewall would normally block.