The trick of retroactively declaring a PDF as PDF/A by appending an incremental update won't work well for signed PDFs, because the PDF reader would recognize the PDF as having been modified since the signature, and when displaying the signed version of the PDF (i.e. removing all incremental updates after the signature) the PDF/A declaration will not be part of it and hence the PDF/A restrictions not be observed by the PDF reader. Put slightly differently: A PDF signature effectively freezes the non-PDF/A nature of a PDF.
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Yes - I think this is only true following https://www.pdfa.org/recently-identified-pdf-digital-signatu...
Those vulnerabilities really have nothing to do with the PDF/A question. PDF signature validators have to check for them regardless of PDF/A, and the issue I raised above is independent of those vulnerabilities.