After you are connected with sqlplus to a container database (which is happening by default), run the connect command inside your sqlplus session.

conn SYS@PMJ AS SYSDBA

Then you'll be required to enter the SYS password.

Another way to switch to a PDB is to change the container:

ALTER SESSION SET container=PMJ;

To go back to the CDB container:

ALTER SESSION SET container=CDB$ROOT;

sqlplus /nolog
connect SNT@PMJ

set linesize 2500
set wrap off
set trimout on
set trimspool on
set long 100000
set pagesize 0
col inmemory_service_name for a10
col table_name for a10
col cluster_name for a20
col cluster_owner for a20
col iot_name for a10
col default_collation for a10
COLUMN first_name FORMAT A10 WORD_WRAP HEADING 'Name' JUSTIFY CENTER

spool C:\buffer\out.txt
@C:\buffer\sql.txt
spool off
exit