Anything passed 2007 uses AES-128 bit encryption, the only method to unlocking is through brute force. If you know your password is say 4 characters long, then go for a brute force but anything larger and you'll be wasting your time.
If it's the 2003 edition, there are programs that will open it.
My advice, unless it's financial documents or very important information, don't password lock the document.
If you're going to lock the file, lock it and then create an unlocked version and store it on a flash drive where you can keep it hidden and safe.