PDF/X
PDF/X is a PDF a printing works can put straight on the press, without ringing anybody to ask what was meant.
What the word means
A file sent to print used to come back with questions. Which red is this. Where is this typeface. 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 a printing works runs as it stands A sheet as it goes to the press The rules behind this word, and the texts they are read from
Glossary
- colour profile
- A file that says what a colour actually looks like. Without one, the same red is one red on your screen and another on a press. A document meant for print carries the profile its colours were chosen against.
- output intent
- The colour profile written into a document to say which press or which screen its colours were chosen for. A printer reads it to know what the numbers in the file are meant to look like, and the rules on documents kept for the long term ask for one.
- CMYK
- The four inks a printing press lays down: cyan, magenta, yellow and black. A screen mixes light instead, so a document going to a press states its colours in inks, not in screen colours.