1. Choose a strong password
Use a long password and share it by a different route than the file.
2. Keep a copy
If the password is lost, the file can't be opened. Keep the original somewhere safe.
3. Removing it
If you know the password, Unlock PDF saves a copy without it.
Good to know
AES-256 is strong, but a short password is easy to guess. Send the password by a different route than the file.
Questions
Can you recover a forgotten password?
No. Unlock PDF needs the password; there is no way round it.
Related: Repair a damaged PDF