Can you elaborate on what problem you have that this rewrite would solve? (Serious question, not trying to be combative.) I can't remember ever having any problem displaying a PDF in macOS, though I don't do anything particularly outlandish with PDFs these days. Does Preview crash frequently for you? (Or does displaying PDFs from within your own apps crash?) I haven't heard of this issue, so just curious what the problem is.
The problem is that writing a parser for a file format as complex as PDF in an unsafe language is just asking for trouble. There were many exploits already that involved a specially crafted PDF file to gain RCE, and there would probably be more, unless they do rewrite it. At this point we should just accept that it's next to impossible to write a 100% safe parser in an unsafe language.
