PDF/A is an ISO standard for PDFs. Documents
you scan to PDF are PDF/A-compliant. You can specify when and whether
you want to view documents in this viewing mode.
Choose Edit > Preferences (Windows) or Acrobat> Preferences (Mac OS).
Select Documents under Categories.
Choose an option for View Documents In PDF/A: Never,
Always, or Only For PDF/A Documents.
You can switch in or out of PDF/A viewing mode by changing
this preference setting again.