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 logical structure of the conforming file shall be described by a structure hierarchy rooted in the StructTreeRoot entry of the document catalog dictionary, as described in PDF Reference 9.6.
Each structure element dictionary in the structure hierarchy shall have a Type entry with the name value of StructElem.
Writers of conforming files should attempt to capture a document’s logical structure hierarchy to the finest granularity possible, making use of the standard structure types for grouping elements, block-level structure elements, paragraph-like elements, list elements, table elements, inline-level structure elements, link elements and illustration elements, as defined in PDF Reference 9.7.4, to the fullest extent possible.
NOTE The explicit description of a document’s logical structure will prove valuable to future efforts to recover the document’s full semantic value for the purposes of rendering or migration to other data formats.