A locked PDF is protected by encryption or password restrictions applied by Adobe Acrobat or another PDF-creation/editing application. If the PDF requires a password to open and you have forgotten or lost that password, the contents cannot normally be accessed without the correct passcode.
An iPhone does not provide a built-in method to bypass or recover a forgotten PDF opening password. The Files app, Preview-like PDF viewers, and other standard iPhone features cannot remove encryption or unlock a password-protected PDF.
If the PDF belongs to you, the practical options are to find the original password, obtain an unlocked copy from the sender/creator, or restore the document from an earlier unprotected backup. Simply changing the file name, opening it in another iPhone app, or saving/sharing it again will not remove the encryption.