It appears that there isn’t a direct activity in UiPath for converting PDFs to the PDF/A-2b format. Instead, you’ll need to use PowerShell or Ghostscript for the conversion. For more details, please refer to the link below.
The problem is that in my company we cannot install these kind of applications (except Powershell), but on the link you sent they only talk about Ghostscript.
I can’t find anythink about converting PDF with Powershell.