If you would like to allow certain users to view documents in their unmasked state for certain Document Types but not for others, you will have to configure privilege overrides for the Document Types on which you wish to allow unmasked viewing.
To configure privilege overrides:
Users belonging to this User Group will now be able to view documents of this specific Document Type in their unmasked state, even if they do not have general privileges to view documents of all Document Types in their unmasked state.
The reverse scenario is also true: if you grant certain users general privileges to view documents of all Document Types in their unmasked state but override this privilege for a certain Document Type, these users will not be able to view documents of the overridden Document Type in their unmasked state. In either case, the override settings prevail.