If you see the “The file couldn’t open in protected view” error while opening Microsoft Excel, the solutions listed in this post may help you resolve the issue ...

You can make Excel always open old spreadsheets in Protected View. Enable or disable Protected View using in-built setting, Group Policy, Registry.

Microsoft's Protected View feature tries to protect you against potentially malicious files and documents. Here's how to use it.

Microsoft Office 2010 includes a new "Protected View" that, by default, will open all Outlook attachments and documents downloaded from the Internet in "read-only" mode.