PDF is an example, but far from the only one. Many file types have executable/scripted portions or simply very complex file formats that have huge attack surfac

PDF is an example, but far from the only one. Many file types have executable/scripted portions or simply very complex file formats that have huge attack surfaces. JPEG XL predictors are Turing complete, given an unlimited image size... Off the top of my head, a lot of old console exploits on the Wii and GameCube revolved feeding malicious save files to games. Same idea, missing bounds checks or whatever when deserializing some field lets you shell the process. Parsing random files from users is just dangerous.