How to use / as sysdba in pluggable database?
First of all, to export from a PDB you shouldn't use SYS for that. It is way over-privileged. You should create a user in the PDB and use that user to connect to the database. You can use the privilege "exp_full_database" to grant the necessary privileges.

Next, you have to specify a connect string to go directly into the PDB (via the relevant service). It is "mypdb_oracle_net" in your example. However, when doing so, you must specify the password for SYS. You can't use OS authentication when doing this.

To solve your problem I would do something like this:

SQLPLUS / AS SYSDBA
ALTER SESSION SET CONTAINER=mypdb;
CREATE USER EXPORT_USER IDENTIFIED BY YOUR_PASSWORD;
GRANT CREATE SESSION TO EXPORT_USER;
GRANT exp_full_database TO EXPORT_USER;

EXPDP EXPORT_USER/YOUR_PASSWORD@mypdb_oracle_net dumpfi......