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What is preflight? A PDF checklist before print

Every mistake found after the plate is made is an expensive one: the plate is remade, paper is wasted, the job runs late. Preflight is the check that catches those mistakes before the file goes to press. Here's what gets checked, and what to do with what you find.

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What does preflight mean?

The word comes from aviation: the checks a pilot runs before take-off. In print the logic is the same: is the incoming PDF ready to print, checked before it goes into imposition. You can do it by hand, but most shops use a program or a ready-made check profile, so less slips through.

Preflight doesn't fix the file; it reports. Not everything it finds is an error, either: some things must be fixed, some only need to be known. Making that distinction is half the job.

1. Bleed

If a background or image touches the edge, there should usually be 3 mm of bleed on every side. A file without it leaves a white sliver at the edge after cutting. Text and logos should also sit at least 3–5 mm inside the trim line (the safe area).

2. Image resolution

For offset and digital printing, images are expected at about 300 dpi at their printed size. What matters is not the image's own resolution but its effective resolution on the page: scale a 300 dpi image to twice its size and it drops to 150 dpi.

Below 150 dpi, most jobs print soft or pixelated. For work viewed from a distance, such as posters and large format, lower values can be acceptable.

3. Colour space: CMYK, RGB and spot colours

Printing uses CMYK inks; RGB images prepared for screens get converted, and vivid colours (bright blues, greens, oranges) can come out dull. Agree up front on who converts, and with which profile.

  • Spot colours: colours meant for a separate ink, such as Pantone, need the right name if they really get their own plate; if not, convert them to CMYK. A spot colour left in by mistake means an extra plate.
  • Black text: small text should be 100% K (black) only. Black text built from four colours looks fuzzy when registration drifts.
  • Rich black: adding some C, M and Y under K in large black areas gives a deeper black; never use it for text.

4. Fonts

Every font used in the PDF should be embedded. A missing font gets replaced on the press side; lines reflow and characters break. If the designer has converted text to outlines there's no problem, but that text can no longer be edited.

5. Total ink, hairlines and overprint

Three more things you can't see on screen but will see in print:

  • Total ink (TAC): the sum of the four colours at any point. The limit depends on the paper, often around 300–340% on coated stock. More than that won't dry and sets off onto the next sheet.
  • Hairlines: lines thinner than 0.25 pt can break up or not print at all.
  • Overprint: a white element accidentally set to overprint disappears in print; a coloured one turns an unexpected colour.

6. Page size and boxes

The PDF's trim size should match the order and stay the same on every page. Bleed belongs inside the page box, crop marks outside the bleed. For multi-page work, the page count has to suit the binding: for saddle stitching, the total is a multiple of 4.

What to do when something turns up

Sending everything back to the designer costs time. A practical split:

  • Fixable in the shop: missing bleed on a flat-colour edge, converting a spot colour to CMYK, switching off overprint on white.
  • Back to the designer: low-resolution images, missing fonts, wrong trim size.
  • Tell the customer: colour shifts from RGB. Saying it before printing is always easier than explaining it after.

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