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.
PDF/A disallows alternative renderings of the document (optional content and layers).
How can I fix it?
Do not use layers, hidden or invisible content 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 optional content and layers in your PDF/A file.
6.1.13 - ISO 19005-1 PDF/A Specification:
The document catalog dictionary shall not contain a key with the name OCProperties.
NOTE: The explicit prohibition of the OCProperties key, which is allowed in PDF 1.5 [19], has the implicit effect of disallowing optional content that generates alternative renderings of a document.