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-1a is one such PDF/X flavor. This page covers:
- What are PDF/x-1a files?
- Which other PDF/X flavors exist?
- PDF/X is just the starting point
- How to create PDF/X-1a files
What is a PDF/X-1a file?
PDF/X-1a files are regular PDF 1.3 or PDF 1.4 files to which a number of restrictions apply:
- All fonts must be embedded in the file.
- All color data must be grayscale, CMYK or named spot colors. The file should not contain any RGB, LAB,… data.
- OPI is not allowed in PDF/X-1a files.
- 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.
- Only a limited number of compression algorithms are supported.
- 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-1a file but that you may not find in regular PDF files:
- There is a separate flag (meaning a switch that is either ON or OFF) that details whether the PDF/X-1a file has already been trapped.
- PDF/X-1a files contain extra operators that define the bleed and trim area.
- The MediaBox defines the size of the entire document
- Either the ArtBox or the 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 which describes the intended printing condition. The output intent is either:
- an ICC color profile. Profiles can be embedded or referenced.
- an output condition identifier, which is simply a text description of the intended print specifications (e.g. FOGRA27).
- There is a flag which indicates that the PDF file is a PDF/X file and which also details what type of PDF/X file it is.
In May 2000, PDF/X was first put to use when Time Inc. processed a Bayer ad that was delivered as a PDF/X-1 file. In the mean time, other organisation have also started getting involved in the definition of PDF/X standards.PDF/X-1a is now an official ISO standard: ISO 15930-1.
There are 2 different PDF/X-1a flavors
- PDF/X-1a:2001 – such a file has to be a PDF 1.3 file.
- PDF/X-1a:2003 – such a file has to be a PDF 1.4 file but it should not contain any transparency and JBIG2 compression should not be used to compress images.
Both flavors share all of the restrictions that apply to PDF/X-1a. The 2003 version simply came along because some newer software applications no longer supported the older PDF 1.3 file format.
Which other PDF/X flavors exist?
Below are all the PDF/X flavors that are either actively used in the market or may become popular in the future.
- 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.
- An updated version of PDF/X-3 which adds among others support for transparency and spot colors.
- 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 from the specific production requirements of a type of printing.
GWG is an industry organization which took the PDF/X standards and then added on top of that a set of rules to cover specific types of printing. There are standard for exchanging files for commercial printing, newspapers, packaging and digital printing. Their current specifications are mainly based on PDF/X-1a.
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, the national trade organizations recommend the use of the GWG standards to exchange files.
How to create PDF/X-1a files
The cheapest solution is probably to purchase and use Adobe Acrobat 7 or later. From version 7 onwards, PDF/X-1a support is build right into Acrobat Distiller, the module that is used to create PDF files.
To convert an existing PDF file to a PDF/X-1a file, you can use a plug-in for Adobe Acrobat Professional. The two plug-ins that I have experience with are Enfocus PitStop and Apago PDF/X Checkup. These two plug-ins can also be used to check if an incoming PDF is a PDF/X-1a compliant file.
- There is also PDF/X-1:1999 – the very first PDF/X standard that was published. It was based on PDF 1.2, didn’t support spot colors and never had a big impact on the market. PDF/X-1a:2001 was the first standard that people actually started using.
- In case you are wondering if there shouldn’t be a PDF/X-2 if there is a PDF/X-1 and a PDF/X-3: actually there is but as far as I know nobody ever used PDF/X-2. Things are already complicated enough as they are.