Yup! That one employee encrypted his/her Microsoft Office documents and forgot the password or worse has left the company Fortunately Microsoft has a solution for situations such as this and its called Microsoft Office 2013 DocRecrypt Tool.
This tool allows admins to unprotect or change the password on password protected OOXML Word, Excel and PowerPoint files.
The tool gives admins who have configured the Escrow key feature options to get access to password protected files. The admin uses the tool and the private key of the escrow certificate to decrypt the file. Once decrypted the admin can choose between creating an unprotected copy of the file and changing the password of the file.
Is this the first you are hearing about this. Well your in good company as I was just made away of this also. I found out that there are some requirements that you need to meet before you can just download and have a go at recovering those documents. You need to set up client computers for password protection removal.
These configurations take place in Group Policy of the machine(s). So you will need the Office 2013 Administrative Template files (ADMX/ADML) and Office Customization Tool
Microsoft has some nice steps on doing this, so please follow this LINK for further details on setting this up.