I was reading the warnings with Tor and was curious about this one inparticular:
Don't open documents downloaded through Tor while online
The Tor Browser will warn you before automatically opening documents that are handled by external applications. DO NOT IGNORE THIS WARNING. You should be very careful when downloading documents via Tor (especially DOC and PDF files) as these documents can contain Internet resources that will be downloaded outside of Tor by the application that opens them. This will reveal your non-Tor IP address.
In what way can opening a DOC or PDF reveal the IP address of a user who's in possession of it (without following a link)? I didn't know just opening a .docx file can communicate arbitrary information to an arbitrary server on the internet?