PDF Preview Issue in File Explorer – Windows 11 24H2 and 25H2

Vimalraj V 20 Reputation points
2026-07-01T09:25:19.9433333+00:00

Hi Team,

We are using Windows 11 24H2 and 25H2 across our environment and are experiencing an issue with PDF file previews in File Explorer.

Error Message:

"The file you are attempting to preview could harm your computer. If you trust the file and the source you received it from, open it to view its contents."

We have already performed the following troubleshooting steps:

Installed all the latest Windows security updates and patches.

Manually unblocked the PDF files via:

  • Right-click file → Properties → General tab → Unblock
    • After unblocking, the PDF preview works correctly.
      • However, this is not a feasible solution as we have a large number of files stored locally and on network shares.
      Used the following PowerShell command to unblock all files in a folder: PowerShell Get-ChildItem "C:\Folder" -Recurse | Unblock-File Show more lines This works temporarily, but whenever a file is replaced or a new file is added to the same location, the issue reoccurs and the command must be run again. Disabled Show preview handlers in preview pane under: 
      - File Explorer Options → View tab

However, this did not resolve the issue.

Modified the following registry setting:

Plain Text

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Attachments

Show more lines

  • Value Name: SaveZoneInformation
  • Value: 1

Despite this change, the issue persists.

This issue is affecting more than 500 Windows 11 devices across our environment. We would like to know if there is a permanent solution, known issue, Group Policy setting, or Microsoft-recommended fix to prevent PDF files from being flagged and blocked from previewing in File Explorer.

Windows for business | Windows Server | Networking | Network connectivity and file sharing
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Answer accepted by question author
Chen Tran 13,030 Reputation points Independent Advisor
2026-07-01T10:21:17.3366667+00:00

Hello Vimalraj,

Thank you for posting question on Microsoft Windows Forum!

Based on the issue description. The possible cause for this issue might be the result of a security hardening change introduced by Microsoft in October 2025 security updates like KB5070960 / KB5066791.

The plausible explanation to this is that to block automated NTLM hash leaks over SMB (which can happen when a preview handler blindly parses external assets like fonts or images via a UNC path), Microsoft flipped the default behavior of the Windows Shell Preview.Specifically, the underlying registry value for URLACTION_SHELL_PREVIEW (180F) inside the Internet Zone (Zone 3) was changed from 0 (Enabled) to 3 (Disabled). As a result, File Explorer strictly blocks the preview handler for any local files carrying the Mark of the Web (MotW) (ZoneId=3/4) and any files residing on Network Shares that Windows natively misclassifies or treats as part of the Internet Zone (often due to accessing shares via FQDN or IP instead of NetBIOS).

The suggestion here is to map network shares to the local intranet zone to see if it works. If the issue is primarily affecting PDFs on your file servers, it means Windows is evaluating those UNC paths under Zone 3 (Internet). Forcing your file servers into the Local Intranet Zone (Zone 1) bypasses the preview block without compromising internet-downloaded file security. You can open your Group Policy Management Console. Navigate to this path Computer Configuration (or User) -> Administrative Templates -> Windows Components -> Internet Explorer -> Internet Control Panel -> Security Page. Open Site to Zone Assignment List. Set it to Enabled and click Show... Add your file servers or domain roots. The value Name should be \server_name or \*.domain.local (Your file share paths) and set the value to 1 (This explicitly maps them to the Local Intranet Zone).

Please note: Performing the above suggestion in a testing environment first before widely rolling out.

For further reference https://learn.microsoft.com/en-us/answers/questions/5728547/how-to-use-preview-feature-given-the-oct-update-th

Hope the above information is helpful!

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  1. Vimalraj V 20 Reputation points
    2026-07-08T06:58:10.4166667+00:00

    Hi Chen Tran,

    We have tried the suggested option for the server, and it is working fine. However, the file preview issue for the local C: and D: drives still persists.

    Is there any other option or solution available to resolve this issue?

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