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.
For example: a PDF file that displays with a low resolution image but when printed prints using a high resolution image.
How can I fix it?
Do not use PostScript 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 in your PDF/A file.
6.2.5 - ISO 19005-1 PDF/A Specification:
A form XObject dictionary shall not contain any of the following:
-- the OPI key;
-- the Subtype2 key with a value of PS;
-- the PS key.
NOTE: In earlier versions of PDF the Subtype2 key with a value of PS and the PS key were used to define arbitrary executable PostScript code streams, which have the potential to interfere with reliable and predictable rendering.