Adobe Acrobat implements the full spec, but modern web browsers implement at least a subset of the functionality in a sandboxed JavaScript runtime environment. Input and output are limited to things one might expect from a fancy PDF form (text input boxes, clickable buttons, things of that nature) but computation-wise, just about anything goes. The playable DOOM PDF therefore implements keyboard input by reading characters typed into a text entry box, or has the user click buttons with the mouse.