PDF/X-1a is the older, stricter print standard: every colour must already be CMYK, grey or spot, and transparency and layers are not allowed, so effects are flattened at export. PDF/X-4 is the newer one: it keeps live transparency and layers and allows ICC-based colour with an embedded output intent profile. Export whichever your printer or print platform asks for; if they accept both, X-4 keeps your effects live instead of flattening them at your desk. ProofSlip then checks the exported PDF on your PC for the problems either way, such as unembedded fonts, RGB images, transparency and ink limit, and shows each one on its page.
This guide explains what the two standards actually require, how to choose, and what to check before you upload.
Check a print PDF on your PC before it goes to the printer.
Get it fromMicrosoft
What is PDF/X in the first place?
PDF/X is a family of ISO standards for exchanging print-ready PDF files. Each part restricts what a PDF may contain so the file prints predictably. The PDF Association’s overview, the technical side and requirements of PDF/X by Peter De Bruyne, sets out the common ground: all fonts must be embedded, there can be no external data, no passwords, no visible annotations and no JavaScript, and every PDF/X file needs an Output Intent that describes the intended printing condition.
The parts differ mainly in what colour and graphic features they allow.
What does PDF/X-1a require?
According to the PDF Association, PDF/X-1a allows only CMYK, grey and spot colour. RGB and device-independent colour are not allowed. Transparency and layers are not allowed either.
In practice that means your layout app does the work at export:
- Colour conversion. RGB photos and graphics are converted to CMYK using the export settings, so the colour decisions are baked in.
- Flattening. Drop shadows, soft glows, feathered edges and any object with opacity are flattened into opaque pieces. This is the step that occasionally creates hairline white gaps or changes how text looks near a transparent object.
- No layers. Optional content is merged.
The result is a predictable, plain file that very old and very new prepress systems all handle the same way.
What does PDF/X-4 allow?
The PDF Association describes PDF/X-4 as based on PDF 1.6. It allows transparency, layers, JPEG2000 compression, OpenType fonts and 16-bit images, and it always embeds the ICC profile of the output intent. Like PDF/X-3 before it, it allows ICC-based colour, so an RGB image can stay RGB with a defined profile and be converted at the printer’s end. A variant, PDF/X-4p, allows the output intent profile to be referenced externally instead of embedded.
The standard itself is ISO 15930-7:2010.
What that buys you:
- Live transparency. Effects are flattened, if at all, by the printer’s own RIP rather than at your export.
- Late colour conversion. Colour can be converted for the actual press condition rather than a guess made at your desk.
- Layers kept intact. Optional content, such as language versions on separate layers, survives the export.
How do they compare side by side?
| Feature | PDF/X-1a | PDF/X-4 |
|---|---|---|
| Colour allowed | CMYK, grey, spot | Also ICC-based colour, such as RGB with a profile |
| Transparency | Not allowed, flattened at export | Allowed, kept live |
| Layers | Not allowed | Allowed |
| Fonts | All embedded | All embedded, OpenType allowed |
| Output Intent | Required | Required, ICC profile embedded (X-4p may reference it externally) |
| Who converts colour | You, at export | Can be the printer, at output |
Which one should you choose?
The rule that never fails: use what the printer or platform asks for. Their prepress system and their checks are the ones that matter.
When you have a choice:
- Choose PDF/X-4 if your design uses transparency, soft shadows or blend effects, if images are RGB and you want the printer to handle conversion, or if the printer says their workflow is built around it.
- Choose PDF/X-1a if the printer, platform or publication asks for it, if you need the colour fully decided before the file leaves you, or if you are sending to a workflow you do not know.
- Ask about the output intent. Both standards need one, and the profile should match the paper and press. A coated sheetfed profile on an uncoated job will mislead everyone.
For book printing specifically, IngramSpark asks for PDF/X-1a or PDF/X-3; our guide to IngramSpark PDF file requirements covers the rest of its list.
See RGB images, transparency and unembedded fonts boxed on the page.
Get it fromMicrosoft
What goes wrong when you export the wrong one?
A few patterns turn up again and again:
- X-1a with heavy effects. Flattening breaks a drop shadow into many pieces, and some viewers show faint lines where the pieces meet. Check a proof before deciding it is a print defect.
- X-4 to an old workflow. If a printer’s system does not handle live transparency well, effects can print differently from your screen. That is why the printer’s instructions come first.
- RGB sneaking into X-1a. Some layout apps convert on export; others refuse to create the file. Either way, check what came out.
- Wrong output intent. A file that conforms perfectly can still print too dark or too heavy if the profile does not match the stock.
What should you check after export?
Choosing a preset is not the same as having a good file. After you export, check:
- Fonts. Every font embedded.
- Colour spaces. No unexpected RGB in a CMYK job, and spot colours named correctly.
- Transparency. Present only if the standard and printer allow it.
- Image resolution. Effective resolution high enough at the printed size.
- Ink limit and rich black. Total ink within the printer’s limit, and no small text in four colour black.
- Size, bleed and safe area. Trim size correct, bleed present where artwork runs off the edge, and text inside the safe area.
ProofSlip checks those things on your PC. Pick a job preset, from business cards to book interiors and paperback covers, and it reports fonts, image resolution, colour spaces and the plates the file needs, along with rich black, hairlines, ink limit, white overprint and transparency. Select a finding to see its page, with the problem boxed and the trim and safe area drawn in. There is no account and no upload, and your files are never changed.
To be clear about the limit: ProofSlip does not claim to validate PDF/X conformance. Your layout app’s export preset and your printer’s own checks decide that. ProofSlip tells you what is in the file and where the problems are, in plain language, before you send it. For a fuller comparison, see ProofSlip vs Acrobat Pro preflight.
Frequently asked questions
Is PDF/X-4 better than PDF/X-1a?
It is newer and more capable, keeping transparency, layers and managed colour. Better depends on the printer’s workflow, so follow their instructions.
Can a PDF/X-4 file contain RGB images?
Yes, as ICC-based colour. The PDF Association notes that PDF/X-4 inherits the PDF/X-3 colour rules, which permit ICC-based RGB. PDF/X-1a does not allow RGB.
Why does my PDF/X-1a file have thin white lines?
They often come from flattening transparency at export. Zoom in on a proof or print a test before assuming it is a defect.
Does ProofSlip fix the file?
No. It is a check, not a fixer. Make changes in your layout app, export again and check the new file.
Try ProofSlip
ProofSlip is a one-time purchase with a free trial in the Microsoft Store; the listing shows the current price and system requirements. Export your next job in the format your printer wants, then run it through ProofSlip before you upload. More detail is on the ProofSlip product page, and our print-ready PDF checklist covers the rest of the handoff.
Catch font, colour and transparency problems before the printer does.
Get it fromMicrosoft
Last updated 24 October 2026. Published by Automata Labs, a software company in Canada.