Initially, when you preview a PDF attachment, you will see the following warn message with ticked check option ahead. If you uncheck the option and click Preview file button, you will not see the warn message any more.
Always warn before previewing this type of file
To restore the warning message, using the following procedure.
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Exit Outlook.
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Start Registry Editor.
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In Windows 7, click Start, type regedit.exe in the search box, and then press Enter.
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In Windows 10, Windows 8.1, and Windows 8, press the Windows key + R to open a Run dialog box, type regedit.exe, and then click OK.
3. Find following registry key:
HKEY_CURRENT_USER\Software\Microsoft\Office\Previewers\PDF
4. Right click on PONTState, and click on Modify.
5. In the Value Data box , input the following digit and click Ok.
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Digit 0 represents to display the warning box
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Digit 1 represents to hide the warning box
