Ghostscript
Ghostscript is a free program that opens a PDF file and says what is written inside it, without deciding what it means.
In more detail
Two programs can read the same page and answer differently, and neither is wrong. A checker answers what reading software would see: it fills in what a page leaves out, because that is what reading software does. Ghostscript answers what the page states, and stays silent where the page says nothing.
That difference is the whole of why it is here. A page that states no cut mark and a page that states one exactly at its edge look the same to a checker and different to Ghostscript, and for a printing works the two are not the same page at all.
What it does on a real document
Every print-ready document created here is put to it. One rule of the print standard cannot be asked of the checker's report at all, so it is put to Ghostscript instead — and when Ghostscript is not on the machine, that rule is named as one that was not checked rather than counted as passed.
The same page is then created a second time by Ghostscript itself, which shares nothing with the program that created the first, and ten points of structure are compared between the two. Two independent programs agreeing about a file is worth more than one program agreeing with itself.
Where it comes from, and why
It was first released in 1988, written by Peter Deutsch, and is developed today by Artifex Software. It reads both PDF and the older page language printers were built around, which is why it ended up inside a great many printers, print servers and conversion tools.
It is free, and anybody may read how it works. That matters for the same reason it matters for the other checker: a file that has to be opened decades from now cannot depend on a company still selling a program decades from now.