PDF/X

Over the last decade there has been a significant growth in the use of PDF for transferring graphic arts content files between sites and between companies. As the growth of PDF use has continued it has become increasingly clear that the PDF file format alone is simply too flexible to enable robust and predictable exchanges of pre-press data.

A family of international standards has therefore been developed to define restrictions on the content and use of PDF files to make print output more predictable. These standards are collectively called PDF/X, each with a suffix to differentiate the specifications defined for different sets of market needs. Each is published in a part of International Standard ISO 15930.

PDF/X-1a

PDF/X-1a is designed to provide the most robust and, to some extent, least flexible delivery of PDF content data. It requires that the color of all objects be expressed in CMYK or spot colors, all prepared for the print conditions under which the final press-work will be run. Elements in RGB and Lab color spaces or tagged with ICC profiles are prohibited. It also requires that all fonts used in the job be embedded in the supplied PDF file and disallows the use of transparency and layers (optional content).

PDF/X-3

PDF/X-3 allows slightly more flexibility in that color managed workflows are also supported: elements in Lab, and RGB or CMYK images with attached ICC source color profiles may also be used, but transparency and layers are still prohibited.

PDF/X-4

PDF/X-4supports color-managed, CMYK, gray, RGB or spot color data, as well as layers and PDF transparency.

PDF/X-4p

PDF/X-4p is based on PDF/X-4, and allows the ICC profile describing the print characterization for which the file was created to be stored external to the file itself. It’s intended to assist in workflows where a very large number of files are composited together, such as catalogs, template document creation and advertising for publication and newsprint.

PDF/X-5g

PDF/X-5g is based on PDF/X-4 and allows one or more graphical elements (e.g. images) to be held in another PDF/X file external to the main file itself. The intent is to enable an OPI-like workflow, where, for instance, images and other graphical elements of pages are designed in parallel with the page itself.

PDF/X-5pg

PDF/X-5pg is based on PDF/X-4 and combines the extensions added in PDF/X-4p and PDF/X-5g.

PDF/X-5n

PDF/X-5n is based on PDF/X-4p, and allows the use of process color spaces other than CMYK, RGB and Gray. Thus it can be used for spaces such as Hexachrome, and for the process spaces used in packaging.

PDF/VT is built on PDF/X

Both the PDF/X-5g and PDF/X-5pg standards are important foundations for workflows using multiple files built around the PDF/VT standard (ISO 16612-2). Here they enable the use of ‘chunking’ workflows. In these images and other graphical elements that are used multiple times are stored in one file, while a series of other files that contain overall page definitions, single-use data and references to the shared data (the ‘chunks’) are supplied to a Digital Front End (DFE). Such workflows allow PDF to be used in a streaming model, where the first pages of a job are printed before the definitions for the last pages of the job have been written into the series of files.

PDF/X processing and creation

Each of the PDF/X standards defines the elements that must be included in a PDF, and those that must not. They also set out how a PDF/X file should be processed by the receiver of the file; if different approaches to overprinting are taken at different sites, for instance, it will not be possible to predict the exact appearance of the final printed piece from a pre-transmission proof, so the standard sets out how overprinting must be performed.

File creators (and the tools that they use) therefore have the responsibility to ensure that the PDF/X files that they create conform to the standard. The various PDF/X standards can, and have, provided significant benefit in reducing the number of ‘bad’ files that a print company receives. But the inclusion of a specification for consistent processing also means that the print company must also play their part. Maximum advantage can only be gained if all components within a pre-press workflow or digital production DFE, including the RIP, apply the PDF/X rules correctly.

Control strips

Global Graphics has released a simple-to-use, free-of-charge overprint control strip for checking that proofs and prints made from files transmitted as PDF/X have rendered correctly.

In a design application, the Global Graphics PDF/X overprint control strip is supplied as an EPS file that can be placed outside the trim or bleed area of a job.

When the job is later converted to one of the PDF/X conformance levels and then proofed or printed, the strip will show whether or not the rendering process in that proofing or pre-press system has been configured in conformance to the PDF/X standards. It may also be placed on a press sheet by an imposition application.

PDF/X overprint

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