A recent story making the rounds tells about how an NSA contractor got caught leaking classified information to the media. The contractor wanted to give information in one or more PDF documents to an online publication (The Intercept). She could have just emailed the PDFs, but she was probably worried that the PDFs might contain some metadata that could link the documents back to her. So she printed the PDF files, scanned the printouts with a desktop scanner, probably destroyed the printouts, and then emailed the scanned pages.
It turns out that many modern printers add little yellow dots to every page they print. If you know how to read them, the dots identify the type of printer, the printer’s serial number, and the date and time the document was printed. The dots are hard for people to see, but the contractor’s scanner was sensitive enough to image them.
So when The Intercept sent some of the scanned pages to the NSA asking them to verify the authenticity, the NSA just had to read the dots, identify the printer, and look in server logs to see who used that printer at the time the dots indicated. They arrested the contractor, and a conviction will probably put her in prison for several years.
To me this is reminiscent of typewriter forensics, like how they convicted Ted Kaczynski (search that linked page page for Unabomber). The FBI had a copy of Kaczynski’s typewritten manifesto. He’d used an old typewriter, the kind where the little arms with the embossed letters strike an ink ribbon to mark the paper. The FBI were able to match those pages to the typewriters they found in Kaczynski’s cabin.
It’s easy to see how this kind of trick could be useful to your character. She could do something like what the NSA did and trace a printout back to a person. Or she might print something on someone else’s printer to incriminate the printer’s owner of something.