If you recently updated your computer to Windows 11 version 25H2, you might have noticed something strange: You can no longer see a preview of your PDF files in File Explorer.

Instead of showing a small preview on the right side, it gives you a warning message. Don’t worry—you didn’t do anything wrong. This is a known issue caused by a recent Windows update. Let's see how to fix them.

 

After installing the Windows 11 25H2 update (KB5066835), many users report that PDF previews in File Explorer’s Preview Pane no longer work. Instead of a thumbnail or page view, users see a warning:

“The file you are attempting to preview could harm your computer…”

This is due to stricter security enforcement introduced in the update.

Why is this change happening?

Microsoft introduced this update to address a security vulnerability that could expose sensitive user credentials. When previewing files with embedded HTML elements like or that point to external locations, there's a risk that NTLM hash data could be leaked. Malicious actors could exploit this behavior to intercept login information — so the update adds stricter controls to prevent that

What Is NTLM?

NTLM (Windows NT LAN Manager) is a legacy authentication protocol developed by Microsoft. It’s used to verify a user's identity when accessing network resources, especially in older Windows environments.

Let me breakdown in simple terms as usual for our blog readers..

NTLM is a way your computer checks who you are when you try to open something on a network — like a shared folder or printer. Think of it like showing your ID badge at work before you’re allowed into a secure room. It helps keep things safe by making sure only the right people can access certain files or devices.

How NTLM Works?

Simple,

• Your computer says, “Hey, I want to open this file.”
• The server replies, “Prove who you are.”
• Your computer sends a scrambled version of your password (called a “hash”).
• The server checks it and says, “Okay, you’re allowed in.”

The cool part? Your actual password never gets sent — just the scrambled version.

Got it? Microsoft has now added this for security. But it breakd half of the world depend on previews!

Let's get it into how to fix. Wait, i need clarity on

What is PDF preview in Windows 11 25H2, Anyway?

When you click on a PDF file in File Explorer and turn on the Preview Pane, Windows usually shows you a small version of the file so you can see what’s inside without opening it. This is called a preview.

But after the update, Windows is being extra cautious and blocks previews for some files—especially ones downloaded from the internet or email.

Why It’s Happening?

Microsoft’s KB5066835 update added a new security layer that blocks previews of files marked with “Mark of the Web” (MOTW). These tags are automatically applied to files downloaded from the internet or stored on network drives.

Affected files include:

• PDFs from email or browsers
• Files on network drives
• Documents flagged with MOT

File Explorer not previewing pdf files since 25H2 Windows 11 update [Solution]

Here are some simple steps you can try:

1.      Unblock the File (Easiest way)

Right-click the PDF file that won’t preview.

1. Click Properties.
2. At the bottom of the window, look for a checkbox that says Unblock.
3. If you see it, check the box.
4. Click Apply, then OK.
5. Try previewing the file again.

Fix 2 : Registry Tweak to Windows 11 25H2 PDF preview fix

What This Registry Tweak Does

This tweak changes a setting in Windows that controls how it handles files downloaded from the internet. Normally, Windows adds a tag called Mark of the Web (MOTW) to these files, which can block previews and trigger security warnings.

By applying this tweak, you tell Windows not to add that tag, which can restore PDF previews in File Explorer.

Important Warning

Disabling MOTW means Windows won’t warn you about potentially unsafe files. This can make your computer more vulnerable to viruses or harmful documents. Only use this tweak if:

• You understand the risks
• You download files from trusted sources
• You have antivirus protection

How to Apply the Registry Tweak (Step-by-Step)

Step 1: Open Registry Editor

1. Press Windows + R on your keyboard.
2. Type regedit and press Enter.
3. If asked, click Yes to allow changes.

Step 2: Navigate to the Right Location

In the Registry Editor:

• Go to this folder:

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

If Attachments is not there, simply right click on Policies folder and Select New Key -> Name it as Attachments.

Step 3: Add or Modify the Setting

1. Right-click in the right panel.
2. Choose New → DWORD (32-bit) Value.
3. Name it: SaveZoneInformation
4. Double-click it and set the value to: 1
5. Click OK and close Registry Editor.

Step 4: Restart Your Computer

This helps apply the change fully.

What Happens Next

After restarting:

• Files you download won’t be tagged with MOTW.
• PDF previews in File Explorer should work again.
• You won’t see the “could harm your computer” warning for previews.

If you ever want to undo this tweak:

• Go back to the same registry location.
• Right-click SaveZoneInformation and choose Delete.

PowerShell Method (Quick & Safe for Advanced Users)

If you prefer using PowerShell, here’s the command to apply the same registry tweak:

Set-ItemProperty -Path "HKCU:\Software\Microsoft\Windows\CurrentVersion\Policies\Attachments" -Name "SaveZoneInformation" -Value 1 -Type DWord

How to Run It:

1. Press Windows + X → Click Windows PowerShell (Admin) or Terminal (Admin)
2. Paste the command above and press Enter
3. Restart your computer

This disables MOTW tagging and restores PDF previews.

Technology can be frustrating, but you’re not alone. This is a common issue, and it’s fixable. Take your time, follow the steps, and you’ll be back to previewing your files in no time.