Preparing a PDF for a print shop

Why 3 mm of bleed exists

Paper is cut in stacks, and a guillotine is not perfect. The blade drifts, the stack shifts, and the cut lands a fraction of a millimetre from where it was aimed. If your artwork stops exactly at the page edge, that drift shows as a thin white line down one side of some copies and not others.

Bleed is artwork extended past the trim line so the cut always lands inside it. Three millimetres is the standard ask in most of the world; some printers want five, and large-format work wants more. It is not a margin and not decoration — it is the tolerance the cutting process needs, and it is why a printer asks for a page slightly larger than the finished piece.

Crop marks are the other half: thin lines outside the trim area showing where the cut belongs. Adding bleed and crop marks after the fact works when the artwork already extends far enough; it cannot invent artwork that was never drawn, so a design laid out exactly to the finished size will gain white bleed rather than more of the picture.

Mixed page sizes are a hard rejection

A print job is imposed — several pages arranged on one large sheet, printed, folded and cut. That arrangement assumes every page is the same size, so a document whose pages disagree cannot be imposed at all. It is one of the most common automatic rejections and it is invisible on screen, because a PDF reader happily displays pages of different sizes one after another.

It usually happens by accident: a document assembled from several sources, or one page exported at US Letter among pages at A4 — a four-millimetre difference nobody notices. Normalising every page to one trim size fixes it. Artwork is scaled to fit rather than cropped, so a page whose proportions differ keeps all its content and gains a small even border — conservative, and better than silently trimming someone's edges.

Flatten the interactive parts

A PDF can contain form fields, comments, stamps and annotations, and whether a given printer renders them is genuinely unpredictable — some print them, some do not, and the setting is buried. A contract that prints without its filled-in answers is the classic expensive mistake, and it is usually discovered after a thousand copies.

Flattening burns each annotation's appearance into the page content and removes the annotation, so what you see is what everyone sees and prints. Links are deliberately left working, since turning a table of contents into inert ink would be vandalism rather than a feature.

One caution: flattening rewrites page content, which invalidates a cryptographic signature — correctly, because detecting exactly that is what a signature is for. A signed document going to print needs the conversation with the printer rather than a flatten.

CMYK and fonts: the parts that must be right from the start

Here is where honesty matters more than capability. Screens are RGB and presses are CMYK, and converting between them is not arithmetic — it depends on the press, the paper and the ink, described by an ICC profile the printer supplies. A tool that converts without that profile is guessing, and produces a file that validates and prints the wrong colours, which is worse than one that fails.

So colour conversion belongs upstream, in the software that laid the document out, or downstream in the printer's own workflow. If the job is colour-critical — a brand colour, a photograph, anything being matched to something else — set the profile in InDesign, Affinity Publisher or Scribus before exporting. Scribus is free and exports PDF/X directly.

Fonts must be embedded, and this one cannot be fixed afterwards either. Embedding requires the font file, which is precisely what is not in a PDF that failed to embed it — nothing can conjure it, and substituting a lookalike reflows the text. Most PDFs written this century by Word, LibreOffice or a decent print driver embed their fonts automatically. Check rather than assume: any PDF reader lists the fonts and says which are embedded.

Two things not to do

Do not compress a print PDF to make it smaller. Compression re-encodes images to screen resolution and converts them to screen colour — invisible on a monitor, and exactly what you do not want on a press. Print files are large because print needs the resolution. If the file is too big to email, send a link.

Do not assume PDF/A means print-ready. PDF/A is the archival profile: it exists so a document renders identically in fifty years. PDF/X is the print profile, and it is what a printer means when they ask for a standards-compliant file. They overlap and they are not the same thing.

Finally: send the file and ask them to preflight it. Most printers will, most will tell you exactly what their workflow wants, and the four problems above are the four they were going to raise first. Five minutes of their time beats a reprint.