PDF is mostly a subset of Postscript. It improves on Postscript in terms of speed and security. In contrast to Postscript, it is not a Turing-complete programm

PDF is mostly a subset of Postscript. It improves on Postscript in terms of speed and security. In contrast to Postscript, it is not a Turing-complete programming language. What's left is a bunch of commands placing graphics objects on pages. That means PDF viewers cannot be DoSed in an obvious way by placing infinite loops in the document. It also means that any page in a document can be directly rendered, without rendering the rest of the document first. That's not possible with Postscript, because Postscript documents are programs that need to executed from the start.