What is the Protected View in Excel desktop? I see the "PROTECTED VIEW" message when opening an Excel document from OneDrive.

Protected View is a read-only mode where most editing functions are disabled. There are several reasons why a file opens in Protected View:

• The file was opened from an Internet location.
• The file was received as an Outlook attachment and your computer policy has defined the sender as unsafe
• The file was opened from an unsafe location.
• The file is blocked by File Block setting in file.
• File validation failure.
• The file was opened in Protected View by using the Open in Protected View option.
• The file was opened from someone else's OneDrive storage.

When opening an Excel document from your OneDrive storage with the Excel desktop app, you will get this "PROTECTED VIEW" warning message:

PROTECTED VIEW: Be careful - files from the Internet can contain viruses. 
Unless you need to edit, it's safer to stay in Protected View.

If you know this document can be trusted, you can click "Enable Editing" button to turn off the protected view.

The picture below shows you how to turn off "Protected View" in Excel desktop when editing an Excel document in OneDrive:

⇒ Managing PowerPoint Documents

⇐ Open Excel Document with Excel Desktop

⇑ Managing Excel Documents

⇑⇑ Office 365 - Frequently Asked Questions