in Production
Print-ready PDFs
A print-ready PDF is more than a file that opens. It is one a printer can run without guessing, and here is what that involves.
Almost every printer now asks for books to be supplied as PDFs. But a PDF that looks fine on screen is not necessarily one a printer can use. A print-ready PDF contains everything the press needs, in the form it needs it, with nothing left to interpretation.
Two files, not one
A book is usually supplied as two separate PDFs: one for the interior and one for the cover.
The interior file has every page in reading order, each at the trim size, plus bleed if any pages have images or colour running to the edge. Pages are usually supplied as single pages, not spreads. The page count normally needs to be even, since every leaf has two sides, and offset printers, who print in signatures of many pages on one sheet, may ask for a count that fits their signatures. Blank pages at the end make up the difference.
The cover file is a single flat spread: back cover, spine and front cover side by side, with bleed on all outer edges. Its width depends on the trim size and the spine width, which in turn depends on the page count and paper. Print-on-demand services generally provide a template.
What makes a PDF print-ready
Fonts are embedded, so the printer does not need to have them. A PDF without embedded fonts may substitute a different typeface, or fail to process.
Images are at a suitable resolution for their printed size, commonly around 300 ppi.
Colour is set up as the printer asks. Covers for offset printing are usually supplied in CMYK, and black text should normally be a single black ink rather than a mix of four. Some printers accept RGB files and convert them themselves, in which case their instructions apply.
Bleed is included where needed, and important content sits within the safe zone.
Crop marks and other printer's marks are included or left off according to the printer's instructions. Many print-on-demand services do not want them; many offset printers do. Where marks are added, they sit outside the bleed.
Nothing is left over from editing: no comments, form fields or hidden layers that could print unexpectedly.
PDF/X
PDF/X is a family of ISO standards for PDFs intended for print. A PDF/X file has to meet certain conditions, such as embedded fonts and defined colour, which rules out many common errors. The two most widely used are PDF/X-1a, which requires colour to be CMYK or spot colours and transparency to be flattened, and PDF/X-4, which keeps live transparency and allows colour-managed RGB.
Professional layout software can export to either. Many printers specify one of them, and if they do, it is the one to use.
Checking the file
Open the exported PDF, not the layout file, and look at every page. Check that fonts are embedded, that images look sharp at full size, that nothing has shifted during export, and that blank pages fall where they should.
Preflight tools in professional PDF software check a file against a set of print requirements and flag problems such as missing fonts, low-resolution images or colour in the wrong mode. Many printers run their own preflight when a file arrives and will report errors, but it is better to find them first.
A clean PDF is still not a proof. Some problems only show in print, so the file check should be followed by a physical proof.