I think the term “system programming” is not sharply defined. I consider e.g. torch and CUDA more towards it as it requires knowledge and understanding of hardware and resources and they are rather providing an interface for the next layer of developers. Is pthreads or openmp written by system programmers or application programmers?

PDFs are if I’m not mistake not turing complete, they omitted those PS features from the standard for various reasons, so you probably can’t write anything reasonable in “PDF” (unless you use embedded JS etc)