Someone built a working Linux environment that runs entirely inside a PDF file. The project uses JavaScript execution built into PDF readers to create a functional operating system. It follows the same approach as Doom-in-a-PDF, which proved PDF specs allow for more computing power than most people realize.
The implementation relies on the JavaScript for Acrobat API, part of the official PDF specification since version 1.7. Document-level JavaScript runs when the file opens, using app.setInterval() to create the continuous execution loops an OS needs. Form fields handle keyboard input. The dataObjects interface provides persistent storage across sessions. Clever combination of features that were probably never meant to run an entire operating system.
Security catch: VirusTotal flags the file as potentially malicious, detecting it as a possible Trojan. Not because it contains actual malware. The same techniques that make Linux-in-a-PDF work are also techniques malware authors use. The security community has noted this paradox before. Legitimate demonstrations expose how much computational power PDF readers actually possess.
The bigger question is whether PDF readers should be Turing-complete at all. The PDF specification can perform any computation given enough time and memory. Fun for running Linux in a document. Less fun when you remember every PDF you open could theoretically run arbitrary code. Maybe document viewers shouldn't double as general-purpose computing platforms.