We tested the PDF encryption feature and found that it uses 128-bit AES encryption on Office 2016, too. This is widely considered secure, and means that Office now uses real, strong encryption to protect your documents when you set a password. Office 2007 switched to the Advanced Encryption Standard (AES) with a 128-bit key. With Office 2007, Microsoft got more serious about security. If you have a document password-protected with Office 2003 or an earlier version, the password can be easily and quickly bypassed with widely available password cracking software.
From Office 95 to Office 2003, the encryption scheme was very weak. Microsoft Office’s password-protection features have gotten a bad rap in the past. How Secure Is Microsoft Office’s Password Protection?
