You indicated that there is an "email thread which provides further details" but there is none provided. Can you please provide more detail to your question?
PD4ML does support PDF/A generation. I cannot verify what version of PDF/A format is supported, but it's important to note that v2 and v3 only add enhancements to what is permissible in a PDF/A document - therefore a PDF/A v1 document actually fully meets the standard for PDF/A v3 format (Remember, this is an long-term archival format.)
Currently PD4ML's site is down so I was unable to further research this issue.
I've sent an email to PD4ML customer support asking for the specifics around their support of specific PDF/A versions. The version of PD4ML that ships with Pega 7 is 398fx4a1 Pro DMS. We have no current plans to upgrade, but we may re-evaluate based on what I hear back about my inquiry.
Here's the response I got from PD4ML customer support re/ my inquiry:
PD4ML supports PDF/A-1b (which refers to "Version 1")
Versions 2 and 3 are extensions of the Version 1 and backward compatible. For example, Version 3 spec allows to embed file attachments to PDF (which was forbidden by the Version 1). So in principle, if PDF file is passed PDF/A-1b validation (means, has no attachments), it passes also PDF/A-3.
I picked an arbitrary PDF/A file, generated by PD4ML and validated it in Adobe Acrobat Pro (with Preflight built-in tool) - it successfully passed PDF/A-2 validation. (My slightly outdated version of Acrobat Pro does not validate PDF/A-3)
Hopefully you got the main idea: PD4ML outputs PDF/A, which can pass PDF/A-3 validation. But if your customer needs a support of PDF/A-2 and -3 extensions, they has to be implemented by us. In the case we need to know what exactly is desired.
We stopped upgrading our iText libraries a while back because they instituted a licensing change that was incompatible with our distribution model. It's possible that the customer could purchase a license for iText that they could use to convert every PDF document generated by Pega to PDF/A-3. This is outside the scope of what the core Pega platform provides however. The version of iText that currently ships with Pega is 2.1.7.