what each one actually requires,
and which to use when.
PDF/X asks whether a file will print correctly at a specific shop. PDF/A asks whether it will still open and look identical in thirty years. PDF/UA asks whether a person using a screen reader can actually use it.
All three work the same way: remove anything the file depends on but does not contain. They differ in what they consider a dependency, which is why a file can satisfy one and fail another.
| Standard | Purpose | Transparency | Colour | Open |
|---|---|---|---|---|
|
PDF/X-1a
ISO 15930-1:2001, revised as ISO 15930-4:2003
|
Print exchange | Not allowed, must be flattened | CMYK and spot colours only | |
|
PDF/X-3
ISO 15930-3:2002, revised as ISO 15930-6:2003
|
Print exchange | Not allowed, must be flattened | CMYK, spot, plus colour-managed RGB and Lab | |
|
PDF/X-4
ISO 15930-7:2008, revised 2010
|
Print exchange | Live transparency permitted | CMYK, spot, plus colour-managed RGB | |
|
PDF/A
ISO 19005-1:2005, 19005-2:2011, 19005-3:2012
|
Long-term archiving | Not allowed in A-1, permitted from A-2 | Device-independent, colour must be unambiguously defined | |
|
PDF/UA
ISO 14289-1:2014
|
Accessibility | Permitted | Unrestricted, but colour alone must not convey meaning |
Every standard above is built on PDF 1.x. There is a newer generation built on PDF 2.0 (ISO 32000-2), and it is worth knowing the names even though almost nobody is asking for them yet.
In production these remain rare. Presses, RIPs and submission portals overwhelmingly still specify the PDF 1.x standards, so if a printer or a portal has named a standard it is almost certainly one of the five above. Treat PDF 2.0 as the direction of travel rather than as something to switch to unprompted, and use it when somebody asks for it by name.
A PDF does not announce its conformance, and opening cleanly proves nothing. Upload one and Filecheck reports the standard it actually meets, plus what would have to change to reach another. Free, no signup, files deleted after processing.
Transparency. PDF/X-1a requires everything to be flattened into opaque artwork before export and permits CMYK and spot colours only. PDF/X-4 keeps transparency live and allows colour-managed RGB, so flattening happens at the RIP at output resolution rather than destructively in your file. Both require embedded fonts and an output intent.
The one they ask for. If nobody has specified anything and they accept it, PDF/X-4 is the sensible modern default because it avoids destructive flattening. Printers who specify PDF/X-1a usually have a reason, often an older RIP, and that is worth respecting rather than arguing with.
They answer different questions. PDF/X asks whether the file will print correctly at a specific shop. PDF/A asks whether it will still open and look identical decades from now. Both work by removing external dependencies, but a file built for archiving is not necessarily suitable for print, and the reverse is equally true.
The letter describes how much is guaranteed. b guarantees visual reproduction only. a additionally requires tagged structure, reading order and Unicode mappings, so the content is machine-readable and accessible. u sits between them, requiring Unicode mappings but not full tagging. PDF/A-1b is the most commonly requested because almost any source document can reach it.
You cannot tell by looking, and the fact that a PDF opens says nothing about conformance. Run it through a validator. Filecheck reports which standard a file actually meets and what would need to change to reach another.