The Publishing PDF

While many industries convert paper documents into PDF files for efficiency, printing and publishing industries rely upon an entirely opposite approach, utilizing digital files to produce high quality prints. As the PDF format is well-suited for prepress workflows, the exchange and quality of PDF files intended for print become critical. Consequently, a standard was developed for this--the PDF/X.

What is PDF/X (Exchange)?

A subset of the PDF standard, the PDF/X was primarily developed by the Committee for Graphic Arts Technology Standards (CGATS) to restrict the use of elements in a PDF file that could compromise the quality and accuracy of the printed file. The standard they helped to establish is divided into two parts: ISO 15929 and ISO 15930. After being withdrawn in March 2008, ISO 15929 stopped being an official standard, though it specifies how to write PDF/X standards. It enables individual industry groups to write specifications that accommodate their own restricted subsets for rare prepress situations. ISO 15930, however, directly outlines how to tailor PDF printing elements in order to produce a proper PDF/X file.

Why The Need For PDF/X?

In a real world example, it is efficient to use a single ad in the PDF format for numerous publication submissions, most of which are done with little communication. This makes an error-free blind exchange vital to producing the desired results. The PDF/X standard minimizes file errors that hinder the printing process (ie. missing images, incorrect colour spaces and unembedded fonts).

Furthermore, control over file processing involves catering PDF elements to accommodate other factors: varied digital press models, different paper stocks, specific printing procedures and industrial specifications. For these factors, a single PDF/X standard becomes inadequate. Thus, in order to better satisfy individual process controls, the PDF/X is further divided into different parts.

Different Types of PDF/X

PDF/X-1a:2003 is published as ISO standard 15930-4:2003 and is the most strict. Printers requesting this type require PDF file elements to be encoded only in CMYK (Cyan, Magenta, Yellow and Black ink) or spot colour codes. With CMYK, it is easier to produce consistent colour reproductions for printing. This is for printers who want to retain maximum control over the printing process. PDF/X-1a prohibits partial transparency and also requires embedding of all fonts used in the PDF file.

PDF/X-3:2003, published as ISO standard 15930-6:2003, is similar to PDF/X 1a:2003 in technical requirements. However, this superset can also contain colour-managed data. It is used where print jobs are optimized for a printer's own specific printing environment. For instance, a PDF/X-3 file is used if printing companies transfer data in RGB colour spaces and convert it into CMYK later on. However, transparency, encryption and layers are prohibited in this standard as well.

PDF/X-2:2003 published as ISO standard 15930-5: PDF/X-2 is more specialized for high-volume publishers. The use of PDF/X-2 requires more communication between both parties. These printers normally need the data required for finer output to be transferred separately from the main file. This type of PDF is designed for use in OPI-like workflows (Open Press Interface specifications use low resolution images as place holders for the higher resolution images needed for better quality).

PDF/X-4 and PDF/X-4p support printing of colour-managed, CMYK, RGB, gray or spot colour PDF files, and there is no layer and PDF transparency restriction as in the previous standards. The second edition of this ISO standard is 15930-7:2010. PDF/X-4p assists in workflows where a large number of files are blended together.

PDF/X-5 is a collection of three PDF/X levels: PDF/X-5g, PDF/X-5pg and PDF/X-5n. The latest ISO edition of this standard is 15930-8:2010. PDF/X-5g allows the use of external files that contain additional graphical elements and is intended for OPI-like workflows. PDF/X-5pg also allows the use of external graphics, while PDF/X-5n supports process colour spaces other than CMYK, RGB and Gray.

One should note that the PDF/X-1a:2001 and PDF/X-3:2002 standards are based on the PDF 1.3 file specification (Acrobat 4.0). Newer versions based on PDF 1.4 (Acrobat 5.0) were approved as ISO standards in 2003: PDF/X-1a:2003 and PDF/X-3:2003. The newest versions that support PDF 1.6 include more advanced characteristics such as layers and live transparency (flattened before PDF/X creation).

The only major setback is that the standard will always be a version behind the current PDF specification.

By using the PDF format itself as a basis, these sectors can further improve the overall quality of published material. Undoubtedly, the PDF will continue to play a significant factor in developing modern technology, regardless of industry and workflow.