The PDF/X-4 file format

PDF is a very versatile file format. Unfortunately, this also means that it is very easy to create PDF files that are completely useless in a prepress environment. The solution to this problem is to define a set of rules that forbid the use of certain PDF functions that are irrelevant for printing purposes and to enforce others that do improve its usefulness in prepress. This set of rules is called PDF/X, a series of well-defined subsets of the PDF standard that promise predictable and consistent PDF files.

PDF/X-4 is one of the newest PDF/X flavors. This page covers:

  • What are PDF/X-4 files?
  • The various PDF/X-4 versions
  • Which other PDF/X flavors exist?
  • PDF/X is just the starting point
  • How to create PDF/X-4 files

What is a PDF/X-4 file?

PDF/X-4 files are regular PDF 1.6 files.

  • The use of transparency is allowed.
  • All color data can be grayscale, CMYK, named spot colors, RGB, Lab or ICC-profile based.
  • Image data can be 8-bit or 16-bit. 16-bit images are still rarely used and may cause issues with quite a few RIPs and workflows.
  • The use of layers is allowed. In PDF/X-4 these are not the ‘regular’ PDF layers (OCG or Optional Content Groups for the techies among us) but the more sophisticated OCCD layers (which actually combine multiple OCGs in a group). If a regular PDF has, for instance, a layer with comments and another one with contact information, both layers could be combined in a single ‘info’ OCCD layer. When I updated this page in early 2008, there weren’t any workflows on the market yet with proper support for handling OCCDs. Meanwhile callas pdfToolbox is the first PDF tool that has proper support for them.

The following restrictions apply to PDF/X-4 files:

  • All fonts must be embedded in the file. Embedding OpenType fonts is allowed.
  • OPI is not allowed in PDF/X-4 files: all image data must be embedded.
  • Compliant files cannot contain music, movies, or non-printable annotations.
  • If there are annotations (sticky notes) in the PDF, they should be located outside the bleed area.
  • The file should not contain forms or Javascript code.
  • Only a limited number of compression algorithms are supported, JPEG 2000 being one of them.
  • Encryption cannot be used.
  • Transfer curves cannot be used.

Next to things that are not allowed, there is also some information that needs to be present in a PDF/X-4 file but that you may not find in regular PDF files:

  • There is an extra line of information which indicates that the PDF file is a PDF/X-4 file.
  • There is a separate flag (meaning a switch that is either ON or OFF) that details whether the PDF/X-4 file has already been trapped or not.
  • PDF/X-4 files contain extra operators that define the bleed and trim area.
    • The MediaBox defines the size of the entire document
    • The ArtBox or TrimBox defines the extent of the printable area.
    • If the file is to be printed with bleed, a BleedBox must be defined. It must be larger than the TrimBox/ArtBox, but smaller than the MediaBox.
  • The file needs to contain an output intent that describes the intended printing condition. The output intent is either:
    • an ICC color profile. Profiles can be embedded or referenced (in such a case when ICC profiles are externally supplied, such a file is called a PDF/X-4p file).
    • an output condition identifier, which is simply a text description of the intended print specifications (e.g. FOGRA27).

PDF/X-4 versions

The PDF/X specifications are evolving standards, which regularly get adapted to deal with newer technologies and applications as well as changes in the graphic arts industry. That is why the full name of PDF/X implementations includes the year they were established.

  • The original PDF/X-4 standard was also referred to as PDF/X-4:2007.
  • Early 2008 PDF/X-4 became an official ISO standard: ISO 15930-7:2008. That version is called PDF/X-4:2008.
  • There have been some complaints about the way fonts and colors are defined in the 2008 specifications. That is why a minor update of the specs is expected to be released in 2010. Those specs will be called PDF/X-4:2010.

Which other PDF/X flavors exist?

Below are other PDF/X flavors that are either actively used in the market or may become popular in the future.

  • PDF/X-1a
    • The first standard, created for black&white, CMYK or spot color jobs.
    • This is a standard that originated in the USA but is also popular in Europe.
  • PDF/X-3
    • To print black&white, CMYK or spot color jobs
    • Files may contain RGB/LAB/… colors in which case the user’s color management system needs to convert these to CMYK.
    • This standard was developed in Germany and Switzerland. Its use also seems to be largely restricted to those countries.
  • PDF/X-5
    • Derived from PDF/X-4, allows external images.

PDF/X is just the starting point

If you think all of the above restrictions make sure that you get perfectly printable PDF files, think again. There are no rules in PDF/X that state that images need to have a certain resolution. A file with 50 dpi images can be a valid PDF/X file yet the printed result will be horrible if used for printing glossy magazines. PDF/X is meant to be a standard which is independent of the specific production requirements of a type of printing.

GWG is an industry organization that took the PDF/X standards and then added on top of that a set of rules to cover specific types of printing. There are standards for exchanging files for commercial printing, newspapers, packaging and digital printing.

If you want to get absolutely perfect PDF files for the type of jobs that you print, head over to the GWG site and check out their specifications. In a lot of countries, national trade organizations recommend the use of the GWG standards to exchange files. The GWG 2015 specifications are based on PDF/X-4.

How to create or process a PDF/X-4 file

Some Adobe applications such as Acrobat 8 and InDesign CS3 include support for the draft-PDF/X-4 specifications. They are not fully compliant since the PDF/X-4 specifications still changed after the release of this software. Acrobat 9 and Creative Suite 4 are fully compliant.

PitStop Professional 7.52 is a preflight tool that can not only check for PDF/X-4 compliance but that can also fix existing PDF files to make them PDF/X-4 compliant. callas software also markets PDF/X-4 compatible tools.

It is a safe assumption that many prepress workflows can cope with generic PDF/X-4 files, even though they may stumble when dealing with some of its advanced features.

Source: prepressure.com

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