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 contains content to be imported from another PDF file. PDF/A does not allow external content.
How can I fix it?
Use Solid PDF Tools to combine the externally referenced PDF document into a single PDF document to alleviate the need for external references.
After you have combined your PDF content into one document in Solid PDF Tools, then File | Save As PDF/A.
6.2.6 - ISO 19005-1 PDF/A Specification:
A conforming file shall not contain any reference XObjects.
NOTE: Reference XObjects refer to arbitrary document content in external PDF files, creating external dependencies that complicate preservation efforts.