PDF/A error
PDF/A error: LZWDecode compression is not allowed
PDF/A forbids the LZWDecode filter. Where it still comes from and how to re-encode with Flate instead.
A stream in the document is compressed with the LZWDecode filter, which PDF/A does not permit. Re-encode the affected streams with FlateDecode, which is allowed and generally compresses better anyway.
ISO 19005-1 clause 6.1File structure
The File structure check group covers 6.1.x — file header, trailer, xref, object framing, limits. The exact sub-clause is reported on the issue itself when the validator raises it.
What causes it
- An old producing application, or an old export preset, defaulting to LZW — it was the common choice before Flate became universal.
- Images imported from TIFF files that carried LZW compression forward.
- A document generated by legacy software that has not been updated in a long time.
How to fix it
- Re-export with Flate (ZIP) compression selected instead of LZW.
- Convert the document: re-encoding a stream from LZW to Flate is lossless and semantics-preserving, so conversion handles it without touching the content.
Frequently asked
Why was LZW excluded?
Its exclusion dates from the era when LZW was patent-encumbered, and it was never reinstated. Flate is unencumbered, better supported and usually smaller, so there is no practical reason to want LZW back.
Does re-encoding degrade my images?
No. Both LZW and Flate are lossless, so the decoded bytes are identical either way.
Check your own document
The validator runs the same engine in your browser and names every failing check. The file is never uploaded.