PDF/A is the Archive suggested practice. The main requirement is the file have no external references, such as referring to fonts that are supposed to be loaded by the PDF processor; instead fonts are to be embedded.
It's likely there is some other flaw that has nothing really to do with PDF/A, but the group that wrote the PDF/A part fixed what the other group didn't.
From Wikipedia on PDF/A
- Audio and video content is forbidden.
- JavaScript and executable file launches are forbidden.
- All fonts must be embedded and also must be legally embeddable for unlimited, universal rendering. This also applies to the so-called PostScript standard fonts such as Times or Helvetica.
- Colorspaces specified in a device-independent manner.
- Encryption is forbidden.
- Use of standards-based metadata is mandated.
- External content references are forbidden.
- LZW and JPEG2000 image compressions are forbidden in PDF/A-1, but JPEG 2000 compression is allowed in PDF/A-2.
- Transparent objects and layers (Optional Content Groups) are forbidden in PDF/A-1, but they are supported in PDF/A-2.
- Provisions for digital signatures in accordance with the PAdES (PDF Advanced Electronic Signatures) standard are supported in PDF/A-2.
- Embedded files are forbidden in PDF/A-1, but PDF/A-2 offers the possibility to embed PDF/A files, allowing archiving of sets of PDF/A documents in a single file. PDF/A-3 allows embedding of any file format such as XML, CAD and others into PDF/A as complete archived objects.
- The use of XML-based XML Forms Architecture (XFA) forms is forbidden in PDF/A-2. XFA is also not allowed in PDF/A-1, because it is used in PDF specification since PDF 1.5. (The preservation of the XFA form data in a PDF/A-2 file may be done by moving from XFA key to the Names tree that itself is the value of the XFAResources key of the Names dictionary of the document catalog dictionary.)
- Interactive PDF forms - Every form field shall have an appearance dictionary associated with the field's data in PDF/A. The appearance dictionary shall be used when rendering the field, the value of a form field shall not be used.