Why a password-protected PDF becomes a problem later
A PDF gets encrypted for a reason at the time — a bank statement, a payslip, a signed agreement someone locked down before sending. The trouble shows up later, when you need to do something ordinary with it: merge it into an application packet, pull out one page, or run it through a compressor. Almost every other PDF tool, on this site or anywhere else, expects an unencrypted file, and will simply refuse to open an encrypted one. Unlocking isn't about bypassing security you don't have rights to — it's the necessary first step before you can do anything else with a file you're already entitled to open.
This only works when you already know the password. A genuine unlock tool decrypts what you give it permission to decrypt; it has no way to guess or crack a password you don't have, and shouldn't try to.
What you're actually trading away
The part people don't expect: an unlocked PDF and the original document aren't quite the same file with the lock removed. Because writing true PDF encryption/decryption at the file-object level is a much deeper operation than most lightweight tools implement, the practical approach is to open each page with the password, render it as a high-quality image, and rebuild a brand-new PDF from those images.
That means the output opens password-free, but its text is no longer selectable, searchable, or copyable — every page is now effectively a picture of the original page. If you need to grab a paragraph of text afterward, that specific capability is gone. Know this before you unlock something you'll want to quote from later.
A sensible order of operations
If your actual goal is to merge an encrypted PDF with others, or pull a few pages out of it, unlock first, then do the second operation on the unlocked copy — encrypted files fail to load in basically every other tool, so this step always comes first in the sequence, not last.
Keep the original encrypted file around after unlocking, at least until you're sure you don't need the selectable-text version for anything. Once you've unlocked and started working from the image-based copy, there's no way to get the original text layer back from it.
When this is (and isn't) the right tool
This is the right move for a file you own or have explicit permission to open, where you just need it in an unlocked state to keep working — most personal and business documents fall squarely into that category.
It's the wrong tool if you're trying to open something you don't have the password for. There's no legitimate way around that here, and there shouldn't be — a tool that could crack arbitrary PDF passwords would be a security liability, not a convenience feature.