"Secure PDF" gets used loosely to describe two entirely different mechanisms: password protection (encryption that gates access) and digital signatures (cryptographic proof of who approved a document and whether it changed). Confusing the two leads to real problems, like assuming a password-protected contract has legal signing weight, or that a signed PDF can't be read by unauthorized people.
What a password actually protects
Password protection encrypts a PDF's content so it can't be opened, or in some cases can't be printed, copied, or edited, without the correct password. It's an access-control mechanism, its entire job is keeping unauthorized people out or limiting what authorized people can do once inside. It says nothing about who created the document or whether its content is trustworthy, a password-protected PDF full of false information is just as password-protected as an accurate one.
This is set up with a tool that adds encryption, and removed the same way. If a file has a password you no longer need enforced (you're the recipient and just want to read it freely, or you set it and forgot to remove it before sharing more broadly), Unlock PDF strips the password restriction, assuming you have the password or the right to remove it.
What a digital signature actually proves
A digital signature applied through Sign PDF doesn't restrict who can open the file at all, a signed PDF can still be completely unprotected and readable by anyone. What it proves is that a specific identity (verified through a certificate) approved this exact version of the document, and that nothing has changed since. Any edit after signing, even a single character, breaks the signature's validity, which is precisely what makes it useful for contracts, approvals, and anything where tamper-evidence matters more than access control.
When to use one, the other, or both
A confidential document that shouldn't be readable by unintended recipients needs password protection, regardless of whether anyone needs to "sign" it. A contract, approval, or legal document needs a digital signature to establish authenticity and integrity, regardless of whether it also needs to stay confidential. Many real documents need both: a signed contract that's also password-protected in transit combines identity verification with access control, each mechanism doing the job the other one can't.