Hello, I am reaching Shanghai Pudong Airport at 23:30 and need to get to Changshu.

No bus or train is available at that time. So you can only take a taxi, and the fare is at least CNY500.
Or you can wait till next day.

Take metro line 2 and transfer to line 4 at Century Avenue, and get off at Railway Station.
Then, walk northwest for 3 minutes to General Bus Station and take a bus to Changshu. The metro fare is CNY7, and the bus fare is CNY42.