Search intent
Combine “password protect PDF” with “send tax return” or “privileged memo.” Expectation is recipient needs a passphrase.
Autoprotect
Set an open password, share the passphrase through a different channel, batch ZIP if you have multiple client PDFs with unique passwords (process one password group at a time).
Limitation
Open passwords are not DRM; determined recipients can still share. Use client portals for highest sensitivity.