Every time a new version of Acrobat applications was released, Adobe also improved the PDF version. As a result, there are many different versions of the PDF format. ISO standard 32000-2 applies to the latest PDF 2.0 version, and it covers 5 subsets of PDF. Let us focus on each of the PDF subsets, starting with the PDF/X standard, which was developed at the request of the graphic arts industry to make document exchange easier.
1 PDF/X - Printing
PDF/X is a subset of the PDF ISO standard. At one point, newspaper publishers and advertisers requested a standard for exchanging vector advertising data. In response, the American National Standards Institute's (ANSI) Committee for Graphic Arts Technologies Standards (CGATS) developed the PDF/X-1.
It is important to note that PDF/X (X comes from eXchange) is not a special version of the PDF format, but a standard that aims to prevent problems that may arise when exchanging documents used in efficient graphic production (in the print and pre-press industry).
PDF/X has a series of printing-related requirements which do not apply to standard PDF files. PDF/X files must contain a special file identification that indicates which PDF/X version they are. In other words, a file can only conform to one specific PDF/X standard, even if it meets all other requirements of another version.
A PDF document created according to the PDF/X standard must meet the following parameters:
- Encryption cannot be used.
- All objects required for processing a PDF document must be included in the document itself.
- All fonts and images must be embedded in the file.
- Output intent must be specified.
- Only a limited number of compression algorithms are supported.
- The file should not contain forms or Javascript code.
- The PDF document must not be password protected or have other restrictions.
- Transfer curves cannot be used.
- Annotations in the PDF should be located outside the bleed area.
- Active content is not allowed in a PDF/X file.
As new PDF versions appear, new versions of the PDF/X standard are also introduced.
PDF/X standard parts:
- PDF/X-1a (2001) The first PDF-based ISO standard. Created for black & white, CMYK, or spot color jobs. Widely used but does not support transparency or layers.
- PDF/X-3 (2002) The main difference compared to PDF/X-1a: it supports ICC (device-independent colors).
- PDF/X-4 (2010) Transparency and layers are permitted.
- PDF/X-5 PDF/X-5 files are regular PDF 1.6 files, developed to offer more flexibility. PDF/X-5 is a set of three conformance levels, each geared towards different workflows. Each conformance level builds on PDF/X-4 or PDF/X-4p.
PDF/X is a widely used and accepted standard that covers all printing methods. For a more detailed list of PDF/X standards, visit the Wikipedia page.
2 PDF/A - Archiving
PDF/A (A stands for Archival) was published as an ISO standard in 2005. Since then, it has been constantly evolving to meet business needs, new technologies, and norms. This archival format based on PDF provides a way to represent electronic documents so that their visual appearance is preserved over time, regardless of the tools and systems used to produce, store, and reproduce the files.
PDF/A is a PDF that forbids certain functions that could impede long-term archiving. The file must meet requirements that ensure reliable reproduction. To make a PDF suitable for long-term archiving, certain precautions need to be taken:
- all required fonts must be embedded within the PDF
- embedded video and audio data are prohibited
- the file does not contain references to external content
- files must not be encrypted with a password, as all content must always be fully available
- the software must also use the XMP format for metadata, etc.
PDF/A format has to be device- and software-independent, self-contained, self-documented, and transparent. It standardizes viewing, printing, and exchange and guarantees technically acceptable PDF documents in the archive. It is compatible with PDF/X, supports digital signatures, and is used all over the world (especially in Europe). PDF/A is more reliable than a regular PDF, and some organizations use it not only for archiving but also for interchange and interoperability.
3 PDF/E - Engineering
PDF/E is another subset of PDF. It is designed to be an open exchange format and is based on the PDF Reference version 1.6 from Adobe Systems. This standard specifies how PDF should be used to create documents in engineering workflows.
Benefits of PDF/E include:
- Reduces the need for expensive, proprietary software
- Reliable exchange across multiple applications and platforms
- Cost-effective and accurate markup capture
- Developed and maintained by the PDF/E ISO committee
- Lower storage and transfer costs
- Self-contained
4 PDF/VT - Variable and Transactional Printing
PDF/VT is an international standard that was released in August 2010 as ISO 16612-2. It defines an optimized format for variable and transactional printing; contents can change for personalization and variable data. This file format enables businesses and organizations to personalize their files such as invoices, billing documents, marketing materials, advertisements, proposals, direct mail, and more.
PDF/VT has the same file extension as PDF and can be opened by the same reader. However, as with any other PDF standard, it has special features that enable an efficient workflow. PDF/VT is based on PDF standards for printing: PDF/X-4 and PDF/X-5. The use of transparency, color, or grayscale, the addition of 8-bit and 16-bit images, and layers in the project are allowed. However, encrypting the file is not allowed.
There are a few different types of PDF/VT:
- PDF/VT-1 Represents complete files. Typically, a PDF/VT-1 file has at least thousands of pages, and each group of pages represents, for example, invoices. All data in this file is embedded.
- PDF/VT-2 A PDF that can link to other PDF objects. This PDF is suitable for documents that contain large amounts of data and for creating references.
- PDF/VT-2s Supports live streaming and allows processing of selected parts of the data.
- PDF/VT-3 Based on PDF/X-6.
A transactional document such as PDF/VT is a suitable choice for businesses that handle and process different data for each recipient.
5 PDF/UA - Universal Accessibility
"UA" stands for Universal Access. This ISO-defined formal subset of PDF is defined by the ISO 14289 family of standards and was published in 2014. It supports universal access and high levels of accessibility for electronic documents. PDF/UA relies on Tagged PDF files.
PDF/UA support is important for people with disabilities who require or benefit from assistive technology when reading electronic content. With these conforming files, assistive technology users and readers are guaranteed equal access to information. Also, with support for PDF/UA, reader software will be able to:
- reliably reflow text onto small screens,
- provide powerful navigation options,
- change text appearance,
- improve search functionality,
- help with selecting and copying text, etc.
PDF/UA requires tagged PDF files and also adds a variety of qualitative requirements. Tagging is not only important for accessibility; it is also important if you want to reuse the content of the PDF file. PDF/UA can be combined with other PDF-based standards.
