For each PDF/A-1 validation issue we try to explain what it means and, if possible how to resolve it. For a more technical explanation of the issue we quote the related section of the ISO 19005-1 PDF/A Specification.
The document specifies Open Prepress Interface (OPI) parameters, softmask alternates (high and low resolution images in the same file) or the intent is not correctly identified.
For example: the PDF file displays with a low resolution image when viewed online but when printed the output prints using a high resolution image.
Final rendering decisions are reader-specific. As with optional content, PDF/A disallows the use of alternate images.
How can I fix it?
Do not use OPI settings when creating your PDF documents.
If you are working with a legacy document, simply open your PDF file in Solid PDF Tools and then File | Save As PDF/A. This will remove prohibited PostScript parameters and use the base images in your PDF/A file.
6.2.4 - ISO 19005-1 PDF/A Specification:
An Image dictionary shall not contain the Alternates key or the OPI key. If an Image dictionary contains the Interpolate key, its value shall be false. Use of the Intent key shall conform to the rules given in 6.2.9.