Okay, hydra might not be the most accurate term (kudzu?) but I have noticed that every time I try to delete a PDF file 2 more take its place. It's a pernicious problem.
Kidding aside , PDF has hung around for 20 years not just because Adobe supported it but also because it fills a need both among producers and users. It's the document format that is the cheapest to produce and use which offers complex layouts, print fidelity, active code, and (we must not forget) security holes.