I knew about postcript (who doesn't lock up office printers by rendering mandlebrot sents?) but was absolutely convinced PDFs were as well. They're not and it seems they quite deliberately weren't as well. [1] - thanks for setting me straight :)

[1] https://www.cs.odu.edu/~zeil/cs390/latest/Public/turing-comp...

Actually, you can have Javascript in PDF nowadays, so...