Opens but cannot print or copy → permissions password. Demands a password to open → open password.
Upload to the decrypt page; the file is processed and deleted straight away.
Ask the sender, or find the record of when you set it.
An open (user) password encrypts the content itself — without it nothing can be decrypted. A permissions (owner) password only restricts printing, copying and editing, while the file opens normally. Almost every service advertising 'PDF unlock' is dealing with the second kind.
Not realistically for AES-256. There are only two ways it opens: the password was weak (digits only, a common word, a birthday), or what you actually hit was a permissions password. Any service promising instant cracking of strong encryption is either a scam or harvesting your documents.
Try common variants first — birthday, name in pinyin, company initials, the year. If it is a permissions password, a decrypt tool removes it without needing the original password at all.
For documents you own or are authorised to handle it is routine housekeeping. Do not use it to bypass someone else's access control.
No — only the protection is removed; pages, bookmarks and forms stay as they were.
Yes. Encryption work needs the server, and the file is deleted immediately after processing.