if you own the interactive PDF companion to Portal 2, "The Final Hours of Portal 2", then please be aware that the file has been compromised with some malicious code injection.
Valve has acknowledged the issue and has taken the store page offline. so if you had launched the PDF recently, do run a system-wide scan for any malware.
this issue was originally reported here.
Note: DC member Deozaan mentioned about the store page being offline in the chat-room, so credit to him for bringing this into our attention.