Read Only

There may be times when you want to encourage users to open a workbook as read-only
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Saving your Workbook

When you save your workbook, select the Tools drop-down and choose General Options
In this dialog box select "Read-only recommended" and press OK
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Read-Only Prompt

The next time you open this workbook you will see the following prompt
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Using this technique for read-only gives you the option when you open the file, and also allows you to quickly toggle between editing and read-only.

Toggle Read Only

There is a built-in command which allows you to quickly toggle a read-only file to editing and back again.
The "Toggle Read Only" command has not been added to any of the built-in ribbon tabs but can be added from the Excel Options, Customise dialog box.
At the top choose "All commands" and scroll down to Toggle Read Only
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Read Only Attribute

You can change the file attribute to read-only using Windows Explorer but this is not as flexible.