We can enable the restricted mode of a PDB either in CDB or PDB.
In CDB
Enabling the restricted mode of a PDB in the root container is the easiest way to do it.
Check Current Status
[oracle@test ~]$ sqlplus / as sysdba
...
SQL> show con_name
CON_NAME
------------------------------
CDB$ROOT
SQL> show pdbs
CON_ID CON_NAME OPEN MODE RESTRICTED
---------- ------------------------------ ---------- ----------
2 PDB$SEED READ ONLY NO
3 ORCLPDB READ WRITE NO
Enable Restricted Mode
Here we use OPEN RESTRICTED FORCE to enable restricted mode.
SQL> alter pluggable database ORCLPDB open restricted force;
Pluggable database altered.
When PDB is MOUNTED
When the PDB is not open, then you don't specify FORCE.
SQL> alter pluggable database ORCLPDB open restricted;
Pluggable database altered.
Check Current Status Again
SQL> show pdbs
CON_ID CON_NAME OPEN MODE RESTRICTED
---------- ------------------------------ ---------- ----------
2 PDB$SEED READ ONLY NO
3 ORCLPDB READ WRITE YES
As you can see, RESTRICTED is now YES.
In PDB
In a PDB, you can enable its restricted mode like a normal, typical database (non-CDB).
Later on, after finishing maintenance works, you may consider to disable restricted mode on the PDB and open to public.