This is the most cursed paragraph
C code can be compiled to run within a PDF using an old version of Emscripten that targets asm.js instead of WebAssembly. With this, I can compile a modified version of the TinyEMU RISC-V emulator to asm.js, which can be run within the PDF. For the input and output, I reused the same display code that I used for DoomPDF. It works by using a separate text field for each row of pixels in the screen, whose contents are set to various ASCII characters. For inputs, there is a virtual keyboard implemented with a bunch of buttons, and a text box you can type in to send keystrokes to the VM.