By default Mendix does not enforce PDF/A compliance and it has to enabled.
Under the hood Mendix uses the Apache FOP engine to generate PDFs and
it’s configuration is stored in the /resources/fop.xconfig file of the Mendix project.
You can enable it in the configuration file by adding the <pdf-a-mode> tag:
<fop version="1.0">
<accessibility>true</accessibility>
<renderers>
<renderer mime="application/pdf">
<pdf-a-mode>PDF/A-1a</pdf-a-mode>
<version>1.4</version>
</renderer>
</renderers>
</fop>
See https://xmlgraphics.apache.org/fop/2.7/pdfa.html
If your PDF contains user generated content, then converting a PDF to a PDF/a compliant document might be a better option.
https://dev.to/codesharing/convert-pdf-to-pdf-a-with-a-free-java-api-4nm7