We are sorry for the delay in response to your query.
As shared above, that unable to find the document password open it. If you are referring to the secured pdf that is protected with password and required to fill the password to open it. Then you need to contact to the admin or the sender of the pdf who owns the rights of it.
Because there isn't anything that Adobe can help you with your document password.
However, if you are struggling to get the application password to Sign In into it. Then we can surely help you resetting it and Signing Into it.