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 contains transparency, image masks or half tones which are not allowed in PDF/A.
How can I fix it?
Do not use sophisticated transparency or half tone graphics 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 adjust or remove the problem and unused parameters in your PDF/A file.
6.2.8 - ISO 19005-1 PDF/A Specification:
An ExtGState dictionary shall not contain the TR key. An ExtGState dictionary shall not contain the TR2 key with a value other than Default. A conforming reader may ignore any instance of the HT key in an ExtGState dictionary.
Use of the RI key shall conform to the rules of 6.2.9.