What does “print-ready” actually mean?

Your ad came back with a note saying it isn't print-ready, and no explanation you can act on. It is not a judgement about the design. It is a short, specific list of mechanical things, and every one of them has a yes-or-no answer.

Why a PDF, and not the file you designed in

A design file is a set of instructions — use this font, link to that photograph, fill this shape with that colour. It only works on a computer that has all the same ingredients. Open it somewhere else and fonts get substituted, linked images go missing, and the layout quietly rearranges itself.

A PDF is the finished result with the ingredients packed inside it. That is the whole reason printers ask for one. It is also why a PDF can still be wrong: packing the ingredients does not check that they were the right ones.

The seven things being checked

Size. The most common failure and the least interesting one. An ad built at the wrong dimensions has to be scaled, which softens every picture in it and moves everything else.

Resolution. A picture has a fixed amount of detail; printing asks for roughly three times what a screen shows. This one has its own page because it is the only problem on this list that cannot be repaired afterwards.

Colour. Screens make colour out of light — red, green and blue. Presses make it out of ink — cyan, magenta, yellow and black. Some colours a screen can show simply do not exist in ink, and the bright ones suffer most. A vivid screen blue can arrive as a flat purple-navy, and nothing went wrong; that was always the closest ink could get. If your brand colour matters, the time to find out what it does in ink is before twenty thousand copies, not after.

Fonts. When a font is not embedded, the printer's computer substitutes something else, and the substitute is a different width — so your carefully set headline rewraps, and a line of type you measured to fit now doesn't. Exporting properly as a PDF handles this automatically.

Bleed and safety margin. Artwork past the trim, important content back from it. Explained here, including when you don't need any.

Black text. Small text should be printed with the black ink alone. When black is built out of all four inks instead, each one prints on a separate pass, and the slight misalignment between passes shows up as coloured fringing around every letter. It looks like a printing fault. It started in the file.

A file can look professional and still fail

This is the part worth knowing if you use a modern online design tool. The paid tiers of several of them export a PDF with crop marks, bleed and ink colours, so the file arrives looking entirely print-ready. What they do not do is let you control the things above: the colour conversion happens on the way out, through a setting you cannot see; black text can be converted into a four-ink mixture; and the bleed is whatever the tool decided.

So the file that announces itself as amateur is rarely the dangerous one. The dangerous one looks correct and is wrong in ways that only appear on press.

Who ends up paying for it

Usually not the person who made the file — and that is exactly why it keeps happening. A bad file is found by whoever receives it, generally at the worst hour of their week, and they either fix it quietly, chase you for a replacement against a deadline, or run it as supplied and absorb the complaint.

None of those outcomes is improved by finding out sooner on their end. All of them are avoided by finding out sooner on yours. The file is still open on your computer, the photographer's number is still in your phone, and nothing is on a press yet.