PDF/X

PDF/X is the PDF standard for the exchange with a print shop. It fixes what a file has to declare, so the shop does not have to guess: the page boxes, the fonts and the printing condition the colors are meant for.

Choose a level

Set it like any other conformance level. In HTML:

<head>
  <meta name="pdf-format" content="PDF/X-4">
  <title>Open day at the press</title>
</head>

In Markdown, with the page setup in the CSS file:

---
title: Open day at the press
format: PDF/X-4
css: print.css
---

PDF/X asks for a document title. Without one, glu warns and writes Untitled.

Output intent

The output intent names the printing condition the colors are meant for. Without further setup, glu embeds the profile Coated FOGRA39 for offset printing on coated paper.

For another condition, ask the print shop for the profile and load it from the companion Lua file:

local frontend = require("glu.frontend")

frontend.add_callback("page_init", "profile", function(doc, page, pagenum, pageinfo)
    if pagenum == 1 then
        local cp = doc:load_colorprofile("PSOuncoated_v3_FOGRA52.icc")
        cp.identifier = "FOGRA52"
        cp.registry = "http://www.color.org"
        cp.info = "PSO Uncoated v3 (FOGRA52)"
        cp.condition = "Offset printing on uncoated paper"
        cp.colors = 4
    end
end)

identifier is the name of the printing condition, colors the number of components of the profile: 4 for CMYK. See ColorProfile for the attributes.

A relative path resolves against the working directory, not against the document. If the file is missing, the run fails with exit code 4 and writes no PDF.

What changes in a PDF/X file

• Page boxes: the file gets a TrimBox and, with a bleed, a BleedBox, see Bleed and crop marks.
• Links: PDF/X has none. glu leaves them out and warns.
• RGB: colors and pictures in RGB count as sRGB, see RGB in print.

Check the file

pdfinfo shows the page boxes, pdffonts shows that every font is embedded:

pdfinfo -box flyer.pdf
pdffonts flyer.pdf

The standard itself is checked by a preflight, such as the one in Adobe Acrobat: choose the profile for PDF/X-4 or PDF/X-3 and run it on the file. veraPDF, which checks PDF/A and PDF/UA, does not cover PDF/X.

To see what each ink prints, look at the plates.