Decide why you need protection
An open password is useful when you want an extra access barrier on a PDF sent to a specific person. It is not a replacement for enterprise access controls, but it is a practical layer for many ordinary sharing scenarios.
Use a strong, separate password
Avoid names, phone numbers, and short words. Use a long phrase that is hard to guess, and do not send the password through the same message or channel as the file when the contents are sensitive.
Keep a safe unencrypted original
If you forget the password, the protected copy can become inconvenient for you as well. Keep the original in a trusted location and create a protected sharing copy rather than making the encrypted copy your only version.
Test the file before sending
Close your PDF reader and reopen the new file. Confirm that it asks for the password and that the pages render correctly after you enter it. The filename alone is not proof that protection was applied successfully.
Password protection is not redaction
If the file contains a number or line the recipient must never see, a password alone is not enough. Once the file is opened, its visible contents are available to the authorized reader. Use real redaction or removal in the sharing copy, then inspect the result visually.
Use a separate channel for the password when it matters
For sensitive files, sending the document and password in the same message weakens the extra barrier. Share the file through one trusted channel and the password through another when practical. It is not absolute security, but it is better than placing the key beside the lock.