A document going by email or WhatsApp passes through more hands than the one it is meant for. A password is the difference between a private file and a forwarded one.
Protect a PDF →Free every day without an account on some tools. Your file is deleted the moment the job finishes.
The file is encrypted rather than marked as protected, so it cannot be opened without the password in any reader. If you have the password and want it gone — for a bank statement you are tired of unlocking every month — the same tool removes it.
AES-256, the standard the PDF specification defines for strong encryption.
No. It is not stored anywhere, which is the point. Lose it and the file is gone.
Yes. Printing, copying and editing can each be turned off separately from the password that opens the file.