Remove the password from an Office file
Do you have a .docx, .xlsx or .pptx that asks for a password to open (and you know it), or that is locked read-only? Here the password is removed and the file downloads free. The file and the password never leave your browser.
Two different «passwords», and it removes both
On an Office file the word «password» means two different things. One is the password to OPEN: the file is encrypted, without the password nobody opens it. The other is edit protection: the file opens and reads fine, but it cannot be changed (read-only). This page recognises on its own which of the two is there and removes it: for the first you need the password (without it, the document is truly unreadable); for the second you do NOT, because the content is not encrypted and the lock is only a label.
The password to open: decrypted here, with your password
A file protected on opening is no longer a ZIP: it is an archive with the document encrypted inside. Office 2013 and later use «agile» encryption (256-bit AES): from your password, with 100,000 rounds of computing, the key that opens the package is derived. Everything happens in your browser: the password is never sent anywhere. You need the real password: here it is not guessed and not tried in bulk (100,000 rounds are made on purpose to make anyone trying very slow). The old encryption of Office 2007-2010 is not supported, and we say so instead of giving a broken file.
Read-only: removed without a password
Edit protection (in Word «Restrict Editing», in Excel «Protect Sheet») does NOT encrypt anything: it only puts a label with a hash inside the file. That is why it can be removed without knowing the password — it is a known weakness of Office, not a trick of ours. Here only the part that holds the label is rewritten and everything else is copied byte for byte, so the document content cannot change.
How it is checked to be right
The decrypted file is compared byte for byte with what msoffcrypto-tool produces (someone else's program that does the same job): they must be identical. Also python-docx and openpyxl read back the text (it must be the same as before) and the protection state (it must be off), and unzip -t checks the produced archive is valid. A file that downloads but does not open is the worst fault, and this test catches it.
What it does not do
If you do not know the password of an encrypted file, here it is NOT guessed: without the password the document cannot be decrypted, and no serious site says otherwise. Removing protection from a file is not bypassing the rights of whoever wrote it: do it only on documents that are yours or that you have the right to change. To shrink a heavy file there is Compress an Office file, and to remove hidden data (author, revisions) there is Clean up an Office file.