Windows 11/10 PDF Preview Pane Not Working? Fix Mark of the Web Blocks, Edge Handler and Thumbnails

Prerequisites and Scope​

Before changing application defaults or unblocking files, confirm how the failure appears in File Explorer so you can target the right fix:

Quick walkthrough

• Supported operating systems: Windows 10 (version 22H2) and Windows 11 (versions 23H2, 24H2, and 25H2).
• Account permissions: Standard user permissions are sufficient to change File Explorer view settings, change default apps, and unblock individual files. Administrator privileges are required to repair Microsoft Edge or change system-wide thumbnail cache settings.
• Symptom check — Security warning in Preview pane: If selecting a PDF displays the 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," your preview handler is working, but Windows is blocking the file because it originated from the internet, an email attachment, or an untrusted network path. You can skip directly to the section on unblocking trusted files.
• Symptom check — Blank space, static icon, or handler error: If the Preview pane remains completely blank, shows only a static PDF icon, or reports "This file can't be previewed because of an error in the PDF Preview Handler," follow the procedure from the beginning to fix your File Explorer settings and repair Microsoft Edge.

Step 1: Enable the Preview Pane and Thumbnail Handlers in File Explorer​

File Explorer relies on two separate switches in Folder Options to render thumbnails in the main file view and interactive document pages in the right-hand Preview pane. If either setting is turned off—or if Windows performance settings switched them off—PDFs will only display generic icons.

Turn On the Preview Pane​

On Windows 11:

• Open File Explorer (Win + E) and navigate to a folder containing PDF files.
• On the command bar at the top, select View and confirm that Preview pane is checked. (You can also press Alt + P to toggle the Preview pane on or off.)
• On the same View menu, choose Medium icons, Large icons, or Extra large icons if you also want to inspect thumbnail icons in the main folder pane; the Details and List views only show small application icons regardless of thumbnail settings.

On Windows 10:

• Open File Explorer (Win + E) and select the View tab on the top ribbon.
• In the Panes group on the far left, click Preview pane so that it is highlighted.
• In the Layout group, choose Medium icons, Large icons, or Extra large icons when testing folder thumbnails.

Enable Preview Handlers in Folder Options​

Both Windows 10 and Windows 11 use the same Folder Options dialog to control whether background preview handlers are allowed to run.

• In File Explorer, open Folder Options:
• Windows 11: Click the See more (...) three-dot button on the top command bar and select Options.
• Windows 10: Click the View tab on the ribbon and select Options (or Change folder and search options) on the far right.
• In the Folder Options window, select the View tab.
• Under Advanced settings, scroll through the list and configure these two checkboxes:
• Uncheck Always show icons, never thumbnails.
• Check Show preview handlers in preview pane.
• Click Apply, then click OK.
• Click a locally created PDF file to see if the Preview pane now loads its pages.

Step 2: Set Microsoft Edge as the Default PDF Handler and Repair the App​

Windows 10 and Windows 11 include a built-in PDF preview handler provided by Microsoft Edge. If another application took over the .pdf file association without registering a compatible 64-bit Windows Explorer preview handler—or if an Edge update left its shell registration incomplete—the Preview pane will fail to render PDFs.

Confirm the .pdf Default App Association​

On Windows 11:

• Open Settings (Win + I) and select Apps > Default apps.
• In the Set defaults for file types or link types search box near the top, type .pdf.
• Click the current default application shown for .pdf.
• In the pop-up window, select Microsoft Edge and click Set default.

On Windows 10:

• Open Settings (Win + I) and select Apps > Default apps.
• Scroll to the bottom of the page and click Choose default apps by file type.
• Scroll down the alphabetical list on the left to .pdf.
• Click the application currently assigned next to .pdf and select Microsoft Edge from the list.

Repair Microsoft Edge to Re-Register Its Preview Handler​

If Microsoft Edge is already set as your default PDF reader or changing the default app does not restore previews, run an in-place repair of Microsoft Edge. Repairing Edge reinstalls its binaries and re-registers the Microsoft PDF Preview Handler in the Windows registry without deleting your browser profile, bookmarks, or saved passwords.

On Windows 11:

• Close all open Microsoft Edge and File Explorer windows.
• Open Settings (Win + I) and go to Apps > Installed apps.
• Type Microsoft Edge into the search bar.
• Click the three-dot menu (...) next to Microsoft Edge and select Modify.
• When the User Account Control prompt appears, click Yes.
• In the dialog titled Repair Microsoft Edge, confirm you have an active internet connection and click Repair.
• Wait for the download and installation process to finish. Microsoft Edge will launch automatically when the repair is complete.

On Windows 10:

• Close all open Microsoft Edge and File Explorer windows.
• Open Settings (Win + I) and go to Apps > Apps & features.
• Scroll down to Microsoft Edge, click it, and select Modify.
• Approve the User Account Control prompt by clicking Yes.
• Click Repair in the installer window and wait for Microsoft Edge to finish reinstalling and re-registering its components.

Step 3: Unblock Trusted PDFs Blocked by Mark of the Web Security​

Starting with the October 2025 Windows security updates (KB5070960 and subsequent monthly cumulative updates for Windows 10 and Windows 11), File Explorer refuses to preview files tagged with the Mark of the Web (MotW). Windows attaches this hidden Zone.Identifier alternate data stream to files downloaded through a browser, saved from an email client, or copied from certain cloud or external locations.

Security Warning: The Preview pane block prevents malicious files from silently connecting to external servers and leaking your Windows NTLM login hashes merely when you click the file in File Explorer. Only unblock PDFs when you recognize the sender or website and trust the document's origin. Never bulk-unblock an entire Downloads folder containing unverified files from unknown sources.

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Unblock a Single Trusted PDF​

• In File Explorer, right-click the PDF that displays the security warning and select Properties.
• Stay on the General tab and look at the bottom of the dialog next to Security: ("This file came from another computer and might be blocked to help protect this computer").
• Check the Unblock box.
• Click Apply, then click OK.
• Click a different file in the folder and then click the PDF again to refresh the Preview pane. (As Microsoft notes in its support documentation for KB5070960, if File Explorer caches the blocked state, you may need to restart File Explorer or sign out and sign back in for the change to take effect.)

Bulk-Unblock a Trusted Project Folder Using PowerShell​

If you have a dedicated folder of trusted PDFs—such as client invoices or engineering drawings downloaded from a known portal—unblocking them one at a time through Properties is impractical. You can strip the Mark of the Web tag from all PDFs in a specific trusted directory using the built-in Unblock-File cmdlet.

• Right-click the Start button and select Terminal (on Windows 11) or Windows PowerShell (on Windows 10).
• Run the following command to unblock all PDF files inside a specific folder, replacing the sample path with your actual folder path:

Get-ChildItem -Path "C:\Users\YourUsername\Documents\TrustedPDFs" -Filter *.pdf | Unblock-File

• If the trusted folder contains subfolders of PDFs that also need to be unblocked, add the -Recurse parameter:

Get-ChildItem -Path "C:\Users\YourUsername\Documents\TrustedPDFs" -Filter *.pdf -Recurse | Unblock-File

• Switch back to File Explorer and select one of the PDFs to verify that the Preview pane now loads the content.

Rollback for Unblocked Files​

Unblocking a local file deletes its Zone.Identifier alternate data stream (ZoneId=3). If you accidentally unblocked a file you no longer trust and want to restore the Mark of the Web block so File Explorer cannot preview it, run this command in PowerShell against the file path:

Set-Content -Path "C:\Users\YourUsername\Documents\UntrustedFile.pdf" -Stream Zone.Identifier -Value "[ZoneTransfer]`r`nZoneId=3"

Verification of Success​

To confirm that your PDF previews are working properly across Windows:

• Open File Explorer (Win + E) and turn on the Preview pane (Alt + P).
• Select a local PDF (or a downloaded PDF you have unblocked) with a single click.
• Verify that the right-hand Preview pane displays the rendered pages of the PDF and provides a scrollbar to read through the document without opening a separate window.

Alternate Methods for Folder Thumbnails and Third-Party Viewers​

While Microsoft Edge provides a built-in handler for the right-hand Preview pane, many Windows 10 and Windows 11 installations still show a static Edge icon instead of a miniature page cover when viewing folders in Large icons mode. In addition, users who rely on Adobe Acrobat Reader or other third-party PDF editors often prefer to keep their editor as the default .pdf application. You can use either Microsoft PowerToys or Adobe Acrobat Reader's shell extension to enable both folder thumbnails and Preview pane rendering.

Method A: Enable PDF Thumbnails and Previews with Microsoft PowerToys​

Microsoft PowerToys includes dedicated File Explorer add-ons that render both first-page folder thumbnails and Preview pane views for PDFs, regardless of which application you use to open PDFs when double-clicking them.

• Install Microsoft PowerToys from the Microsoft Store or via Windows Package Manager by opening Terminal or PowerShell and running:

winget install Microsoft.PowerToys

• Open PowerToys from the Start menu.
• In the left navigation sidebar, expand System Tools and select File Explorer add-ons.
• Under the Preview Pane section, locate Portable Document Format (.pdf) and toggle the switch to On.
• Scroll down to the Thumbnail Icon Preview section, locate Portable Document Format (.pdf), and toggle the switch to On.
• Restart File Explorer (or sign out and back in) for the new thumbnail and preview handlers to load.

Compatibility Note: Microsoft notes that enabling the PowerToys PDF Preview Pane handler may conflict with PDF attachment previews inside classic Microsoft Outlook. If Outlook PDF previews stop working after enabling PowerToys, turn off Portable Document Format (.pdf) under the Preview Pane section in PowerToys and leave only Thumbnail Icon Preview turned on.

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Method B: Enable Thumbnails in Adobe Acrobat Reader​

If you use Adobe Acrobat or Adobe Acrobat Reader as your default PDF viewer instead of Microsoft Edge, Adobe disables its Windows Explorer thumbnail handler by default.

• Open Adobe Acrobat or Adobe Acrobat Reader.
• Press Ctrl + K (or select Menu > Preferences, or Edit > Preferences on older builds).
• In the Categories list on the left, select General.
• Check the box labeled Enable PDF thumbnail previews in Windows Explorer.
• Click OK and wait a few seconds while the Adobe installer configures the 64-bit shell extension in Windows.

Troubleshooting Common Failure Modes​

Network Share PDFs Still Show the "Could Harm Your Computer" Warning​

Symptom: PDFs stored on an internal company NAS or Windows file server cannot be previewed, even though they were created locally and do not have a Mark of the Web checkbox in their file Properties.

Cause: When you access a network share using an IP address (such as \\192.168.1.50\share) or a fully qualified domain name that Windows does not recognize as part of your local domain, Windows classifies the path under the Internet security zone. Under the October 2025 security hardening rules, File Explorer blocks previews for all files on Internet Zone shares.

Security Warning: Only add internal servers you control to the Local intranet zone. Adding an untrusted remote server to the Local intranet zone relaxes security restrictions for all files hosted on that server and allows automatic Windows credential negotiation.

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Fix:

• Press Win + R, type inetcpl.cpl, and press Enter to open Internet Properties.
• Select the Security tab, click Local intranet, and click the Sites button.
• Click the Advanced button.
• In the Add this website to the zone box, enter your file server path (for example, \\192.168.1.50 or file://nas-server.local) and click Add.
• Uncheck Require server verification (https🙂 for all sites in this zone if it prevents adding a standard SMB share path.
• Click Close, click OK, and click OK again to close Internet Properties, then restart File Explorer.
• Rollback: To reverse this change, reopen inetcpl.cpl > Security > Local intranet > Sites > Advanced, select the server address in the Websites list, and click Remove.

Cloud-Synced PDFs in OneDrive Only Show Generic Icons​

Symptom: Local PDFs show thumbnails and previews, but PDFs inside your OneDrive folder fail to generate thumbnails or show only a small icon inside the Preview pane.

Cause: Files configured as "online-only" through OneDrive Files On-Demand carry the offline file attribute and are not stored locally on your drive until accessed.

Fix: Right-click the PDF file or parent folder in OneDrive and select Always keep on this device. Once OneDrive downloads the full file and displays a solid green checkmark icon next to it, File Explorer can generate the thumbnail and preview.

Thumbnails Remain Blank After Enabling Handlers (Corrupted Thumbnail Cache)​

Symptom: The Preview pane works, and a thumbnail handler (such as PowerToys or Adobe) is enabled, but existing PDFs in folders still display blank white icons or stale app logos.

Cause: Windows caches folder thumbnails in hidden thumbcache_*.db database files. If Windows previously cached a blank icon for a file, it may not regenerate the thumbnail automatically.

Fix:

• Press Win + R, type cleanmgr, and press Enter to open Disk Cleanup.
• Select your system drive (usually C:) and click OK.
• In the Files to delete list, uncheck everything except Thumbnails.
• Click OK, then click Delete Files when prompted.
• Restart Windows Explorer by pressing Ctrl + Shift + Esc to open Task Manager, finding Windows Explorer under Processes, right-clicking it, and selecting Restart.