Wikileaks published CIA “Scribbles” projet source code to watermark Microsoft Office documents, and track them through web beacons
Thusly modified documents will connect with a server thereby allowing someone to track it.
▻https://wikileaks.org/vault7/#Scribbles
▻https://wikileaks.org/vault7/document/Scribbles_v1_0_RC1-User_Guide/page-4/#pagination