10.12.2025

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PDF/A standards at a glance – What you need to know about the sub-standards

“Should I use PDF/A-1, -2, -3, or -4 – and which level: a, b, or e?”
We often hear this question from our customers. The answer is not entirely straightforward, as the various PDF/A standards and their sub-standards differ fundamentally in terms of their requirements and possible applications. In this article, we explain the differences, highlight strengths and weaknesses, and help you make the right decision for your document processes.

What PDF/A standards are there?

PDF/A-1: The origins of the series of standards

The first version of the PDF/A standard, PDF/A-1, was published in 2005 – at a time when PDF was still based on an Adobe specification. The aim was to create a format that was particularly suitable for reliable long-term archiving. This meant deliberately limiting the functionality of the format to ensure that archived files could still be reliably opened and read decades later.

The most important features of PDF/A-1

Based on PDF Reference 1.4.
Limited features: No transparencies, no layers, no JavaScript. These were deliberately not included in the standard to avoid potential compatibility issues.
Fonts must be embedded to ensure long-term displayability.
Metadata and color conformity: Files must contain metadata (based on XMP), colors must be clearly defined (e.g., as ICC profiles).
No embedded files or multimedia elements such as videos may be integrated into the PDF file. Forms, comments, and notes are only permitted to a limited extent.
No encryption is permitted—an important detail to ensure permanent readability.
Compression only for selected objects and no LZW or JPEG2000.

PDF/A-1 meets basic archiving requirements, but is limited in terms of modern features and flexibility.

PDF/A-2: New possibilities with the further development of the format

Six years after PDF/A-1, the second part of the standards series followed in 2011: PDF/A-2. The most important innovation was certainly the fact that this part of the standard could now be based on PDF as an ISO standard, ISO 32000-1 (corresponds to PDF 1.7). Adobe’s implicitly applicable XMP definition had also become ISO standard 16684 in the meantime. The further development of the PDF format also allowed newer functions to be approved for PDF/A.

What’s new in PDF/A-2?

Transparencies allowed: While PDF/A-1 excluded transparencies, they are now permitted because their display is now technically precisely defined, enabling long-term use.
Use of layers: PDF/A-2 enables the integration of layers, which is particularly advantageous for CAD applications and geodata.
Size restriction removed: Instead of the 200-inch limit (approx. 5.08 m) in PDF/A-1, PDFs can now be larger – up to 300 kilometers in size through the use of user units.
Advanced image compression: JPEG2000 images and OpenType fonts are supported, which can reduce file sizes and improve display quality.
Digital signatures: PDF/A-2 proposes for the first time a clear reference to signature standards and digital signatures to ensure their correct application in long-term archiving.
Embedded files: Additional files can be embedded in documents that themselves comply with PDF/A standards.

PDF/A-2 is a significant upgrade from the first version and is often chosen as the standard for new archiving projects – especially when modern features such as transparencies or digital signatures are required.

PDF/A-3: Flexibility through embedded files

In 2013, PDF/A-3 took another step toward flexibility: The only new feature here was the ability to embed any files. You just have to mark the relationship between the embedded files and the container PDF/A-3: previous version or other representation.

However, there are numerous useful applications for packaging any files in PDF/A-3.:

2D PDF views of a design with embedded original 3D geometry
Invoice as PDF and stored invoice data as XML in ZUGFeRD or XRechnung format
Originally existing non-PDF/A file but with valid digital signature embedded in the derived PDF/A file

PDF/A-3 does not guarantee the quality of embedded files in any way. Company-wide regulations must be established to prevent uncontrolled growth within the company.

PDF/A-4: The new standard with simplified rules

In November 2020, PDF/A-4, the latest part of the standards series to date, was published based on the revised PDF 2.0 standard (ISO 32000-2:2020). Numerous simplifications were made in this part to make it easier to use. It was also a long time coming: the standardization of 3D objects in PDF from the perspective of long-term archiving.

Important new features in PDF/A-4

Drastic simplification of standard levels: The jungle of different levels (a: accessible, b: visually correct, u: Unicode extension) has been reduced. Now there are only two levels: f (flat/2D PDFs) and e (engineering/3D PDFs).
Integration of 3D objects: PDF/A-4 allows standardized 3D formats such as U3D and PRC to be inserted. Other formats such as JT or STEP are not currently included, but are being considered for future standard extensions.
Tagging: PDF/A-4 only encourages tagging of PDFs, but does not impose any specific requirements. However, detailed standardization is provided by the PDF/UA standard, which is designed to enable people with visual impairments to access PDFs. Tagging also forms the basis for machine evaluation of PDFs.**
Unicode connectivity: Unlike previous standards, the specification of Unicode characters is now mandatory. This means that either the font used is a Unicode font, or it must also have the name of the corresponding Unicode character for each character used. This facilitates machine reading and ensures better accessibility – but can also lead to complications, as older files may no longer be PDF/A-4 compliant.
Signatures for long-term archiving: Digital signatures were also permitted in the previous parts of the standard. However, Part 4 provides greater clarity. In Part 3, many rules were still found in Annex B. Now these rules are an integral part of the actual standard. In particular, long-term validation (LTV) signatures are recommended in order to be able to verify the authenticity of a signed file even after a certificate has expired. Such a signature consists of a set of timestamps and validation information, which is in turn signed. A certificate usually expires after three years.

It is important to note that a PDF file can only be PDF/A-4 if it is also PDF 2 compliant.

What is Tagging?

Tagging refers to the internal marking of elements such as structures, language, artifacts, word breaks, or alternative texts in a PDF file. This information enables better machine readability and accessibility—important criteria for Level A.

In the first part of the PDF/A standard, a file only had to be marked as “tagged” globally to meet Level A. However, since the completeness and accuracy of tagging is difficult to verify, simply setting the flag was often misleading and of no practical use. We therefore advise our customers not to artificially adapt existing PDFs to Level A. Instead, it is easier to achieve Level A directly using modern tools such as professional PDF export functions from MS Office.

What is the significance of the levels?

A key feature of the PDF/A standards are the so-called levels, which define the degree of requirements for a PDF document. These levels help to distinguish between basic and advanced requirements for archiving documents:

Level A (Accessible) – from PDF/A-1:

The highest standard, ensuring complete conformity of a PDF. All objects in the document must be internally marked (tagging) so that their meaning is semantically defined. This is particularly important for the machine readability of documents, which in turn benefits people with disabilities who have content read aloud to them digitally. In addition, all characters used must be assigned a unique Unicode name—even if the font used is not a Unicode font itself. However, creating such files is time-consuming and often only feasible with high-quality source documents.

Level B (Basic) – from PDF/A-1:

This level is reduced to the essentials: clear visual reproducibility. This means that the layout and content of the PDF can be accurately displayed regardless of subsequent technical developments. Additional requirements such as tagging or Unicode compatibility are not applicable. This level covers most archiving needs and remains the most common choice.

Level U (Unicode) – from PDF/A-2:

With the introduction of PDF/A-2, a new middle ground was added: Level U. It is aimed at users who value consistent Unicode compatibility without implementing the additional requirements of tagging. Among other things, this makes it easier to evaluate stored texts automatically without having to mark all objects in the PDF completely semantically. In practice, this is often a pragmatic solution for companies that need Unicode but do not want to invest in tagging.

Level F (Flat) and E (Engineering) – from PDF/A-4:

In the latest standard, PDF/A-4, the previous level system has been greatly simplified. Here, a distinction is made only between Level F (Flat) for pure 2D PDFs and Level E (Engineering), which specializes in PDF files with 3D content. This reduction is intended to provide clearer guidance and reduce the complexity of choosing the appropriate level.

However, level e (PDF/A-4e) is mandatory for 3D PDF. Accordingly, only the 3D formats U3D and PRC are permitted in PDF/A-4. JT and STEP as streams in PDF will therefore have to wait a little longer. In order to keep this 3D component under control, a number of innovations have been added that will be hardly noticeable to the average user:

3D uses annotations. In this respect, 3D and rich media annotations must now be permitted.
Javascript and actions related to 3D are permitted.
JavaScript should only be executed interactively. Automation is not permitted. However, this cannot be verified independently and is only a requirement for a PDF processor.
All action events must be created centrally in an action dictionary.

The choice of the right level depends entirely on your requirements. If you simply want to ensure the visual readability of your documents, Level B is sufficient. However, for documents that are optimally machine-readable or accessible, you should consider the advanced options offered by Levels U or A.

Which PDF/A standard is right for you?

The decision on the right PDF/A standard depends primarily on your requirements:

If you do not want to use modern features such as transparencies or embedded files, PDF/A-1 is sufficient.
If you need more flexibility, e.g. for image compression or signatures, then PDF/A-2 is the right choice.
If you work with files that contain additional information in the form of attachments, PDF/A-3 is a good choice—but be careful with regard to the quality of the attachments.
If your company wants to rely on future-proof technologies such as 3D content, then you should switch to PDF/A-4 – bearing in mind that this is not always possible seamlessly due to the Unicode requirement.

The complexity of PDF/A standards may seem daunting at first glance. But with the right support and a clear strategy, you can make your document processes efficient and future-proof. Give us a call or send us an email—we’ll help you find the right solution for your archiving needs!

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