Every time a new version of Acrobat applications was released, Adobe would improve the PDF version as well. As a result, there are many different versions of the PDF format. ISO standard 32000-2 is relevant 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, developed at the request of graphic technology with the aim to achieve an easier exchange of documents.
1 PDF/X – Printing
PDF/X is a subset of the PDF ISO standard. At one point, a standard for exchanging vector advertising data was requested by newspaper publishers and advertisers. Thereby, 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 tries to avoid problems that may arise when exchanging documents necessary for efficient graphic production (in 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, which says which PDF/X version they are. In other words, a file can only conform to a single specific PDF/X standard, even if all other requirements of another version are met.
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 PDF document must not be password protected or have some 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, so do different versions of the PDF/X standard.
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 and layers.
- PDF/X-3 (2002) The main difference opposed 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, all geared towards different workflows. Each conformance level expands 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 mechanism for representing electronic documents in a manner that preserves their visual appearance over time, independent of the tools and systems used for producing, storing, and reproducing the files.
PDF/A is a PDF that forbids certain functions which could impede long-term archiving. The file must meet requirements that guarantee reliable reproduction. In order to make PDF long-term archive ready, certain precautions need to be made:
- 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, works for digital signatures, and it is used all over the world (noticeably more in Europe). PDF/A is more reliable than a regular PDF, and some organizations do not just use it 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 based on the PDF Reference version 1.6 from Adobe Systems. This standard specifies how the PDF should be used to create the documents in engineering workflows.
Benefits of PDF/E include:
- Reduces requirements for expensive & proprietary software
- Trustworthy exchange across multiple applications and platforms
- Cost-effective and accurate means of capturing markups
- Developed and maintained by the PDF/E ISO committee
- Lower storage and exchange costs
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, billings, marketing materials, advertisements, proposals, direct mailing, 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 allow 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, adding of 8-bit and 16-bit images, and layers in the project are allowed. But, it is not allowed to encrypt the file.
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 are representing e.g. invoices. All the data that can be found in this file is embedded.
- PDF/VT-2 A PDF that can link to other PDF objects. This PDF is perfect for documents that contain large data and for putting up a reference.
- PDF/VT-2s Supports live streaming. Allows processing for some parts of the data.
- PDF/VT-3 Based on PDF/X-6.
A transactional document, such as PDF/VT, makes a perfect choice for businesses that deal and transact with a variety of data per 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 of interest to persons 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,
- transform text appearance,
- improve search engine functionality,
- aid in the selection and copying of text, etc.
PDF/UA requires tagged PDF files, but also adds a variety of qualitative requirements. The tagging is not just important for accessibility – it is also important if you want to re-purpose the content of the PDF file. PDF/UA can be combined with other PDF-based Standards.