PDF/A Explained: Versions, Conformance Levels, and How to Convert
PDF/A is the ISO standard for long-term archiving of PDF documents (the "A" stands for "archival"). Its guiding principle is that a PDF/A file is fully self-contained. Every font, color profile, and other resource needed to display it is embedded in the file. A document opened decades from now should look the same as it did on the day it was created, regardless of how much PDF software has changed.
The first part of the standard was published in 2005. Adoption was slow at first but has since become widespread, especially in the European Union, where many governments and municipalities now require PDF/A instead of standard PDF for official documents.
Why PDF/A Has Evolved
Part 1 was built on an older foundation. It's based on PDF 1.4, which was dated even then but was widely supported by PDF vendors. Features introduced afterward, such as transparency, aren't allowed. Many believe these limits slowed adoption.
Part 1 also prohibits attachments. The reasoning was archival: a PDF shouldn't depend on outside software to open an attached file. In practice, many real-world documents need associated files to make sense, which made Part 1 impractical for those use cases.
Parts 2 and 3 are based on PDF 1.7. Features that were off-limits in Part 1 became legal, including JPEG 2000 compression, transparency, and attachments. Parts 2 and 3 also introduced a new conformance level, Level U.
PDF/A Conformance Levels
Each PDF/A part comes in different conformance levels, which define how much a file must include to be compliant.
| Level | Stands for | What it requires |
|---|---|---|
| A | Accessible (or "All") | Meets every requirement of the standard, including structure information (tagging) for accessibility |
| B | Basic | Reliable visual reproduction of the document only |
| U | Unicode | Everything in Level B, plus Unicode equivalents for all text. Available in Parts 2 and 3 |
- Level A is the most complete but the hardest to achieve. An unstructured PDF can't be automatically converted to include tagging, which confused many users and likely slowed adoption.
- Level B is the most popular choice, since it guarantees visual fidelity without the overhead of structure information.
- Level U was designed as a middle ground. It makes text searchable and extractable through Unicode mapping without requiring the logical structure that Level A demands.
The Four PDF/A Standards
PDF/A-1
The original standard and still the most commonly used, but also the most restrictive. Because it's based on PDF 1.4 (published by Adobe in 2001), it doesn't support JPEG 2000, layers, or attachments. Transparency was also excluded: although PDF 1.4 supported it, it was considered too new at the time.
PDF/A-2
Based on PDF 1.7 (ISO 32000-1:2008), PDF/A-2 adds transparency, layers, and improved image compression (JPEG 2000 and JBIG2). It also allows attachments, as long as the attached files are themselves PDF/A.
PDF/A-3
Nearly identical to PDF/A-2, with one key difference: it allows attachments of any file type. This is what makes PDF/A-3 the foundation for hybrid formats like ZUGFeRD, where machine-readable XML is embedded inside a human-readable PDF invoice.
PDF/A-4
Sometimes called "PDF/A-NEXT," PDF/A-4 was published in November 2020 as ISO 19005-4:2020. It aligns PDF/A with PDF 2.0, the latest version of the PDF ISO standard.
Which PDF/A Is Right for Your Compliance Needs?
There's no single "best" PDF/A. The right choice depends on what your documents need to do and which regulations apply to them.
| If you need... | Consider |
|---|---|
| Maximum compatibility with legacy archives and long-established mandates | PDF/A-1b |
| Transparency, layers, and better compression, with a visual-fidelity guarantee | PDF/A-2b |
| Searchable, extractable text without full tagging | PDF/A-2u |
| Embedded files of any type, such as XML for e-invoicing (ZUGFeRD) | PDF/A-3 |
| Accessibility compliance through full structure tagging | Level A of any part |
| Alignment with PDF 2.0 | PDF/A-4 |
Always check the specific requirements of your regulator, industry, or customer. Many mandates name a particular part and level, and a file that is "PDF/A" in general may still fall short of what they require.
PDF/A is only one of several PDF standards, though, and it isn't always the right one. Print production workflows, for example, may call for PDF/X, and newer projects may benefit from PDF 2.0 itself. To see how these standards compare and which one your workflow needs, read our next article: PDF/A vs. PDF/X vs. PDF 2.0: Which PDF Standard Does Your Workflow Actually Need?
Ready to build PDF/A conversion into your own application? Adobe PDF Library SDK preserves metadata, graphics, and text formatting during conversion and can produce ZUGFeRD-conformant output. Browse our GitHub repository for code samples.