If you have not recorded your recipient passwords, encrypted PDF's cannot be opened. However, if you prescribe to a two password PDF, you can use your other password.
Two Password PDF - You can add two kinds of passwords to protect your Attach Plus PDF documents. You can add a Recipient (Document Open) password so that only users who have the password can open the document. You can also add a Sender (Permissions) password so that only users who have the password can open and change the permissions for the document. Tip: It is a good habit to assign a "Sender" Password, later you can bypass looking up Recipient password when opening old e-mail attachments. This also helps in an emergency with forgotten Recipient passwords.