Unlocking takes a PDF you can already open and writes out a copy without the encryption, so you stop typing the password every time you look at it. You supply the password; it is used to decrypt the file in your browser and is never sent anywhere. This is not a password cracker and cannot be: without the correct password there is no way to read the contents, which is the property that makes encryption worth using in the first place. The practical need is usually a document you legitimately own — a bank statement, a payslip, an insurance policy — that arrives encrypted by policy and then has to be opened repeatedly, or fed to another tool that cannot handle encrypted input. Decrypting once and storing the result somewhere already secure is often the more sensible arrangement.
This needs the password
Decryption requires the key, and the password is the key. There is no bypass, no reset and no vendor override — a properly encrypted PDF whose password is unknown is unreadable, and any service claiming otherwise is either guessing weak passwords or not telling you the truth.
That is a feature. The same property that makes a forgotten password unrecoverable is what makes encryption meaningful when you are relying on it to protect something of your own.
Why remove encryption at all
Because many documents are encrypted by an institution's policy rather than by your judgement about the risk. Statements arriving with a date-of-birth password, payslips with an employee number — these are protecting the email in transit, and the protection is often actively unhelpful once the file is in your own storage.
The other reason is tooling. Encrypted PDFs cannot be merged, compressed, searched by desktop indexers or processed by most automation. Decrypt once, keep the result somewhere already protected, and the document behaves like a document again.
Think about where the decrypted copy goes
Removing encryption removes a layer of protection, and the file is only as safe afterwards as the place you keep it. On an encrypted disk in a personal folder, that is usually fine. In a shared drive or a synced folder others can browse, it is not.
Note also that owner-password restrictions on printing and copying are lifted along with the rest, since they are part of the same structure. And do the decrypting before any other processing — everything else in this toolkit needs an unencrypted file to work with.