- Choose a Signature from the Sign As menu.
- Enter the Password for the selected digital signature
- Select an Appearance, or choose Create New Appearance.
- Enable Lock Document After Signing if this option is available. Select it only if you are the last recipient to sign the document. Selecting this option locks all fields, including the signature field.
In case it doesn't work you will have to lock the document using Acrobat application (Download Adobe Acrobat free trial | Acrobat Pro DC).
Please refer to this KB doc. for help Acrobat Help | Securing PDFs with passwords.
Regards,
Aadesh