A high-schooler who goes by the online handle of ading2210 has released LinuxPDF, software that runs Linux within a PDF file. This comes after his earlier release of DoomPDF, software that runs the video game Doom within a PDF file. The demonstration version of LinuxPDF can be accessed using a Chromium-based web browser here.
Web browsers use HTML, CSS, and Javascript to create the interactive web pages we see across the Internet today. A subset of those languages are available in modern PDF browsers to execute code.
TinyEMU is a small RISC-V system emulator running Linux, and the uncompiled TinyEMU code with Linux OS can be downloaded in a 15 MB file. This is small enough to be run within a PDF browser without much difficulty, especially when viewing that PDF file within a web browser as linked above.