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 key <<Metadata>> is required for PDF/A-1b Invalid or non-compliant metadata
6.7.2 - ISO 19005-1 PDF/A Specification:
The document catalog dictionary of a conforming file shall contain the Metadata key. The metadata stream that forms the value of that key shall conform to XMP Specification. All metadata properties embedded in a file shall be in XMP form except for document information dictionary entries that have no XMP analogues, as defined in 6.7.4. Properties specified in XMP form shall use either the predefined schemas defined in XMP Specification 4, or extension schemas that comply with XMP Specification 4, and 6.7.8. Metadata object stream dictionaries shall not contain the Filter key.
NOTE 1: The explicit prohibition of the Filter key has the implicit effect of preserving the contents of XMP metadata streams as plain text that is visible to non-PDF aware tools.
NOTE 2: An extension schema is any XMP schema that is not defined in XMP Specification.