Can this open a file whose password I do not have?

No. It removes a password you know — it does not break one you do not. There is no back door here, and a tool that claimed otherwise would be lying to you.

Why do I need this at all?

Mostly to stop retyping a password every time you open a document you already have rights to, or to make a file printable and searchable again.

Unlock PDF

Remove a password you already know, so the file opens without prompting.

Needs a server connectionnew

Needs the same qpdf service as Protect PDF. This removes a password you know — it does not break one you do not.

How to unlock pdf

Choose the password-protected PDF.
Enter the password you already know.
Press Unlock and download a copy that opens without prompting.

Questions

Can this open a file whose password I do not have?: No. It removes a password you know — it does not break one you do not. There is no back door here, and a tool that claimed otherwise would be lying to you.
Why do I need this at all?: Mostly to stop retyping a password every time you open a document you already have rights to, or to make a file printable and searchable again.

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Protect PDF

Lock a document with a password so it cannot be opened without one.

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Redact PDF

Cover a region and flatten the page so the text underneath is genuinely gone.

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Compare PDF

Word-level differences between two revisions, with every change listed.

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