I don't want to send editable file or native file (i.e. Ai.) to client. I read some articles and found out that people send pdf file to client. Do I need to add password to it? because when I open pdf file on illustrator, I still can edit it and see all the layers.
You are assuming that the client has illustrator and understands how to use it. It depends on what the files are used for, too. If I was sending a PDF to a printer, I leave it editable, in case the printer needs to adjust something before running the job.
If you are really concerned about your client adjusting your files afterwards, you can send as EPS, or send a PDF with the "Make PDF editable in Illustrator" unchecked when you create the PDF file.
Password protecting the PDF only limits people opening the file at all or not being able to print from PDF.