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Fix File Explorer PDF Preview Security Warning

Explains how to resolve the File Explorer warning that a PDF file could harm your computer when previewing PDFs with Foxit installed.

Answer

Issue Description
After installing the October 2025 security updates (e.g., KB5066835), Windows File Explorer is unable to preview certain file types, such as PDF files and Office documents. Instead, it displays the following warning:
"The file you are attempting to preview may harm your computer. If you trust the file and the source from which you received it, open it to view its contents."


This issue primarily affects files downloaded from the internet and saved locally, as well as files located on network paths. Switching to alternative PDF preview programs does not resolve the issue.

Cause of the Issue
Microsoft introduced a new security mechanism in the October 2025 updates that prevents File Explorer from previewing files tagged with the Mark of the Web (MOTW) label. This label identifies files originating from the internet or untrusted network locations, restricting their preview to protect users from potential threats.

Solutions
The following solutions will help you remove the MOTW tag and restore file previews in File Explorer. Only apply these solutions to files or locations you trust to avoid potential security risks.

1. Unblock Individual Files
For single files, you can manually remove the MOTW tag:
1)Right-click the file (e.g., a PDF or Office document) and select Properties.
2)In the General tab, locate the “Security” section at the bottom.
3)Check the box labeled Unblock.
4)Click Apply to save the changes.
5)Open Task Manager (Ctrl + Shift + Esc), locate Windows Explorer, and select Restart to apply the changes .If the changes still do not take effect, please restart your device.

Or, you can use command to do above:
1)Open PowerShell with administrative privileges.
2)Run the following command to unblock a specific file:

Unblock-File -Path "C:\Users\UserName\Downloads\example.pdf"

or

Get-ChildItem "C:\Users\UserName\Downloads\example.pdf" | Unblock-File

Replace "C:\Users\UserName\Downloads\example.pdf" with the path to the specific PDF file.
3)Open Task Manager, locate Windows Explorer, and select Restart to apply the changes.

 

2. Unblock Files in Bulk Using PowerShell
For multiple files in a folder, use PowerShell to remove the MOTW tag in bulk:
1)Open PowerShell with administrative privileges.
2)Run the following command to unblock all files in a specific folder:

Get-ChildItem -Path "C:\Users\UserName\Downloads" -Recurse | Unblock-File

Replace "C:\Users\UserName\Downloads" with the path to the folder containing the files.
3)Open Task Manager, locate Windows Explorer, and select Restart to apply the changes.

 

3. Unblock Files from Network Locations
To unblock files located on network paths, add the network location to trusted sites:
1)Open Control Panel > Internet Options (or search for "Internet Options" in the Start menu).
2)Go to the Security tab and select Local Intranet or Trusted Sites.
3)Click Sites and add the shared folder address, for example:

for example: \\192.168.1.100\SharedFolder or file://192.168.1.100/SharedFolder

(To add non-HTTPS addresses, uncheck the box labeled “Require server verification (https:) for all sites in this zone” at the bottom.)
4)Confirm the settings and close the window.
5)Open Task Manager, locate Windows Explorer, and select Restart to apply the changes.

Important Notes
These solutions should only be applied to files or locations that you trust. Avoid unblocking files from unknown or untrusted sources to prevent potential security risks.
By following these steps, you can restore the file preview functionality in Windows File Explorer for trusted files, ensuring a smooth workflow while maintaining system security.