That's apparently how 4chan got hacked a while back. They were letting users upload PDFs and were using ghostscript to generate thumbnails. From what I understand, the hackers uploaded a PDF which contained PostScript which exploited a ghostscript bug.

Yes but the primary issue was that 4chan was using over a decade old version of the library that contained a vulnerability first disclosed in 2012: https://nvd.nist.gov/vuln/detail/CVE-2012-4405