ISO 32000-1 represents the formalization and publication of the complete PDF 1.7 edition of the Adobe PDF Reference and an open international standard. The ISO committee (TC171/SC2/WG8) spent many years producing a standard that was technically identical to the previous Adobe PDF 1.7, but had undergone an extensive rewriting process to clarify numerous items. This standard is the foundation for all future generations of PDF standards.

With “Adobe’s PDF” now standardized, the next move by the committee was to begin work introducing features that they had been wanting in PDF for years. Some examples include the inclusion of geospatial or GIS data, black point compensation for richer color rendition, and improved tagging and structure. At this time, ISO 32000-2 is under development and due to be published sometime in 2015.

The first of the PDF subset standards, PDF/X, focused on the needs of print professionals, graphic designers, and creatives by providing specifications for the creation, viewing, and printing of final print-ready or press-ready pages. PDF/X provides guidelines affecting critical aspects of printing, such as color spaces, font embedding, and the use of trapping. It also restricts other content—such as embedded multimedia—that does not directly serve high-quality print production output.

PDF/X-1a (ISO 15930-1), the first of the family, was published in 2001. It specified a subset of PDF that required that all fonts were embedded, no annotations were included, and the only colorspaces allowed to be used were DeviceCMYK, DeviceGray, and Separation. It also required that the file specify whether it has been trapped or not. In 2002, PDF/X-3 (ISO 15930-3) was introduced; it built on PDF/X-1a but allowed for color-managed colors via colorspaces such as ICCBased and Lab. These original versions of PDF/X-1a and PDF/X-3 were based on PDF 1.3. In 2003 there were updates to both PDF/X-1a (15930-4) and PDF/X-3 (15930-6), that brought them in line with PDF 1.4, but they continued to disallow transparency.

It took a while for the print industry to understand the benefits of and concerns about the use of transparency in PDF, so it wasn’t until 2008 that the first version of PDF/X-4 (ISO 15930-7) was published. The version brought PDF/X-3’s color-managed model for PDF to a version of PDF (1.6) incorporating transparency, better compression (e.g., JPEG2000 and ObjectStream; see Cross-reference table), and more. However, a few issues were discovered in that original 2008 release, and it was superseded by an update in 2010.

PDF/X-1a, PDF/X-3, and PDF/X-4 all represent completely self-contained files. All fonts, colorspaces, and images must be inside the PDF—no external references are allowed. While that is certainly the normal case for printing, there are situations where the ability to refer to external content or external resources can be beneficial, such as in the variable and transactional data worlds. For those industries, PDF/X-5 (ISO 15930-8) was created to provide a standard that enables either single files or sets of multiple files utilizing external references.

In 2003, representatives from the US government approached Adobe Systems about their need to create a subset of PDF that would be more reliable and consistent than what PDF producers were generating at the time. They needed this reliability to be able to properly maintain PDF documents in their archives for 10, 20, 50, or more years.

PDF/A-1, published in 2005 and based on PDF 1.4, represents a standard for the creation, viewing, and printing of digital documents for the purpose of long-term preservation. These documents are completely self-contained with embedded fonts and consistent color, and without any encryption, enabling them to serve as final documents of record. No references to external content are allowed since those items may not exist in the future. In addition, XMP-based metadata is required to ensure that the file is self-describing (see XMP).

PDF/A-2, published in 2010 and based on PDF 1.7 (ISO 32000-1), brings with it many requested capabilities such as transparency and improved compression utilizing JPEG2000 and object streams. By using ISO 32000-1 as the base standard, PDF/A-2 became the first PDF subset standard to be entirely ISO-based. One feature that was requested by many, but did not make it into PDF/A-2, was the ability to have arbitrary attachments, such as XML data. In order to provide a solution for those workflows, the committee (ISO TC171/SC2/WG5) produced PDF/A-3 in 2012.

Each of the PDF/A standards comes in at least two conformance levels, a and b. The a conformance level can be thought of as the “all” or “accessible” level, as it requires conformance with the complete set of requirements for the standard, including that the file be tagged and structured for accessibility (see Chapter 11 for details). The b, or “basic,” level of conformance is commonly used by simpler content such as scanned documents or documents whose original digital source is no longer available. PDF/A-2 and PDF/A-3 introduced a third conformance level that can be seen as being in between the others: level u, for “Unicode.” It requires that all text in the file can be mapped to Unicode.

Although PDF had seen some basic usage in the engineering market all along, the introduction of support for optional content, 3D, and measurements caused a significant uptake of the format amongst architects, engineers, construction professionals, and product manufacturing teams.

PDF/E was a direct result of the engineering community’s desire for a specification that built on top of PDF/A. It focuses on their needs around the exchange of documentation and drawings in the supply chain for document sharing or streamlined review and markup. It specifies requirements for PDF that make it more suitable for building, manufacturing, and geospatial workflows by supporting interactive media, animation, and 3D. Because one of the key use cases for the standard was to enable sharing of content, it allows for the use of encryption and digital rights management.

Published in 2007, PDF/E-1 is based on PDF 1.6. PDF/E-2, which is under development by ISO TC171/SC2/WG7, is expected to be published in 2014 and will be based on ISO 32000-2.

While PDF/X-4 and PDF/X-5 address the majority of the print production industry’s needs, those working with variable and transactional printing needed some specific additions. In their high-volume workflows involving bank statements and business invoices, the inclusion of rich metadata and identifiable document parts was a necessity. In addition, many parts of these documents are reused, and an optimal way to identify them and reuse them in the printing process was required. PDF/VT, which is based on PDF/X, was published in 2010 and addressed these needs.

There are three flavors of PDF/VT. PDF/VT-1 is a completely self-contained file format based on PDF/X-4, while PDF/VT-2 allows for individual form XObjects to be referenced in other files and is built on PDF/X-5. There is also a special case of PDF/VT-2 called PDF/VT-2s that can be driven entirely in a stream, rather than requiring actual writing to files.

Many governments around the world have laws that require that their publications be accessible to all of their people, regardless of any disabilities they may possess. PDF has long had features (especially via tagging and structure) that enable any document to comply with the general-purpose international accessibility standards, but until recently no PDF-focused standard existed to give concrete direction.

PDF/UA-1, published in 2012, is based on ISO 32000-1 and provides a set of guidelines for creating PDF files that are universally accessible and thus more readable for people with disabilities such as vision impairment or limited mobility.

There are a few other PDF-related standards to be aware of.