PDF/A error: transparency is not allowed in PDF/A-1
PDF/A-1 forbids transparency entirely — soft masks, transparency groups and any blend mode other than Normal — because its rendering was not considered reliably reproducible when ISO 19005-1 was written. Either flatten the transparency before export, or target PDF/A-2 or later, which permit it.
The Transparency check group covers 6.2.8 transfer functions, 6.4 soft masks/blend modes/alpha. The exact sub-clause is reported on the issue itself when the validator raises it.
What causes it
- Drop shadows, glows, feathered edges and other effects from a design application, all of which are implemented with soft masks.
- A logo or figure placed with a transparent background.
- Blend modes such as Multiply used for highlighting or watermarks.
- A transparency group on a form XObject, sometimes present even when nothing visibly looks transparent.
How to fix it
- Flatten transparency at export time. Most design and print workflows have a flattener precisely for this.
- Target PDF/A-2 or PDF/A-3 instead, which allow transparency — but check that the receiving side accepts them.
- Convert with gopdfrab, which rasterizes the affected content as a last resort. That produces a conformant file at the cost of resolution-dependent output, so check the result before shipping it.
Frequently asked
PDF/A-1 is based on PDF 1.4, and PDF 1.4 introduced transparency. Why is it excluded?
Because ISO 19005-1 selects a subset of PDF 1.4, and transparency was judged not to render predictably enough across implementations at the time. Later parts revisited that decision.
Can I just delete the alpha channel?
Removing transparency changes the appearance wherever it was doing something. Flattening composites it against the background instead, which preserves what the page looks like — that is why it is the recommended route.
How do I tell whether rasterizing lost anything?
Run the fidelity check: it renders the input and the output and reports a per-page comparison, so content loss is visible rather than assumed.