PDF/X
PDF/X is a PDF a printing works can put straight on the press, without ringing anybody to ask what was meant.
In more detail
A file sent to print used to come back with questions. Which red is this. Where is this font. How far past the edge does the picture run. Every question cost a day.
PDF/X is the list of answers, written into the file itself. The colours are stated in a way a press understands, and the document names the press they were chosen for, which is called the output intent. Every letter drawn is inside the file. The page says where the blade cuts and how far the printing runs past that line. Nothing is left to be asked.
What it does on a real document
A press sheet carries two rectangles a reader never sees. One is the finished size, after cutting. The other is slightly larger, and says how far the colour must run so that a cut a hair off centre still leaves no white edge.
The engine refuses to write such a file when anything the rule asks for is missing. A refusal at the moment of writing costs nothing; the same file found wrong at the printing works costs a print run.
Where it comes from, and why
The rules were written by the printing trade itself rather than by the makers of the software. They are published as an international standard numbered ISO 15930, in several parts, each one stricter about a different thing.
The oldest still in common use is PDF/X-1a, which accepts nothing but inks. PDF/X-3 came next and lets a document keep colours described against a profile. PDF/X-4 is the current one: it accepts a page built in layers that show through one another, which is how design software actually works, instead of asking for the page to be flattened first.
See it on a finished document
A sheet prepared for the printing works A sheet as it goes to the press The page that goes further on this word