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.
A PDF/A-1 file must be identified as such by the presence of a certain entry in its XMP document metadata. In order to be able to retrieve this entry reliably, it is essential that all syntactical requirements for storing metadata are followed precisely. In early implementations of PDF/A tools this was not always the case due to several reasons.
For more information go to the PDF Association website and from the Publications page, search for the relevant Technical Note publication.