Remove the password from a PDF
The PDF that asks for a password every time you open it, or the one that opens by itself but will not let you copy or print: here you get the same document back, without the lock. You need the password you already know: nothing is guessed here. And the file never leaves your browser.
A PDF has two locks, and they are different
The first is the open password: without it the file cannot be read, and there is nothing anyone here can do about that. The second, far more common, is the owner restrictions: the PDF opens on a double click, but the program refuses to print, to copy the text or to sign it. In that second case the open password is empty, so there is nothing to type: you drag the file in and the lock falls away. If instead the PDF asks for a password when opening, this page asks you for it, and it has to be the right one.
Why do it here and not on the first site you find
Because that kind of file is almost always yours: the payslip, the medical report, the bank statement, the contract. Every online service doing this job asks you to upload the document to its servers, and from that moment on you no longer know where it is. Here the PDF is opened, decrypted and rewritten inside the page, by your own browser: no network request goes out, and you can check that by pulling the plug before dragging the file in.
What happens to the document
The file is read in full, every encrypted piece is decrypted with the key worked out from your password, and then the PDF is written again from scratch without the protection part. The content is untouched: text, images, fonts and pages stay as they were, and the document you download opens in any reader. Before handing you the file, the page reads it back and demands to find no trace of protection left: if it finds any, it downloads nothing and tells you.
If you do not know the password
Then this tool is no use to you, and it is only fair to say so up front: it does not try passwords over and over and it guesses none. That is not shyness, it is the difference between taking off a lock you have the key to and picking it. Two things you can still try: drag the file in as it is, because a great many «protected» PDFs really only carry the restrictions and open with an empty password; and ask whoever sent it for the file again, because the password is often written in the same email.
What it does not do
It does not open PDFs with company licence protection (the various Adobe DRM systems): those have no password, they have a permission that lives somewhere else, and you are told instead of being strung along. It does not remove a digital signature: on the contrary, rewriting the file invalidates one, so a signed PDF is better left alone. It does no OCR and changes the content in no way. If your PDF is not protected but you want to work on it, the rest of the tools are already here: Merge PDF, Split PDF, Sign a PDF and Extract text from PDF.