Fix: PDF Files Cannot Be Previewed in Outlook or Windows Explorer

A recruitment customer reported that PDF files saved into SharePoint folders or attached to Outlook emails would no longer preview without a security warning. The user had to manually unblock each file before opening it, which broke their workflow because they relied on PDF preview to triage candidate documents quickly. The issue resolved temporarily after a fix but kept returning every few days. Here is what we found and how to fix it permanently.

Symptoms

When clicking a PDF in Outlook (Reading Pane preview), Windows Explorer (Preview Pane), or a SharePoint synced folder, users see one of:

This file came from another computer and might be blocked to help protect this computer.

Or:

The publisher could not be verified. Are you sure you want to run this software?

Or PDF preview simply fails silently with no preview pane content. After right-clicking the file, choosing Properties, ticking Unblock and applying, the file previews and opens normally. The issue returns the next time a new PDF is downloaded or synced from SharePoint, OneDrive, or an external email.

Affected Software

• Application: Windows Explorer Preview Pane, Outlook Reading Pane, Adobe Reader, Microsoft Edge PDF preview
• Platform: Windows 10 and Windows 11 (especially with Group Policy enforced)
• Related Services: SharePoint Online, OneDrive for Business, Outlook (any version)

Cause

Windows tags every file downloaded from “untrusted” zones (Internet, restricted sites) with a Mark of the Web (MOTW) using an Alternate Data Stream called Zone.Identifier. The Attachment Manager then enforces extra security checks before previewing or executing those files. SharePoint and OneDrive sync clients write the MOTW onto every synced file by default, which is why even your own company documents get treated as untrusted. The Attachment Manager setting Do not preserve zone information in file attachments tells Windows to skip writing this MOTW, allowing PDFs to preview without warning.

Resolution

The fix can be applied per-user via local Group Policy (or Intune for fleet-wide deployment) or via a registry edit.

Option 1: Local Group Policy (single workstation)

1. Press Windows + R, type gpedit.msc and press Enter.
2. Navigate to User Configuration > Administrative Templates > Windows Components > Attachment Manager.
3. Open Do not preserve zone information in file attachments, set it to Enabled, then click OK.
4. Open Inclusion list for low file types, set it to Enabled, then in the Options box add .pdf (you can chain extensions with semicolons, for example .pdf;.docx;.xlsx).
5. Open an elevated Command Prompt and run gpupdate /force, then sign out and back in.
6. Open a previously blocked PDF from a SharePoint folder. Preview should now work without the security warning.

Option 2: Registry edit (no Group Policy editor on Windows Home)

1. Press Windows + R, type regedit and press Enter.
2. Navigate to HKEY_CURRENT_USERSoftwareMicrosoftWindowsCurrentVersionPoliciesAttachments. Create the Attachments key if it does not exist.
3. Create a new DWORD (32-bit) value named SaveZoneInformation, set its value to 1.
4. Create a string value named DefaultFileTypeRisk, set its value to 1808 (low risk default).
5. Sign out and back in for the change to take effect.

Option 3: Intune for fleet-wide rollout

Use a Settings Catalog profile with the policy Administrative Templates > Windows Components > Attachment Manager > Do not preserve zone information in file attachments = Enabled, scoped to your user groups. The same Inclusion list for low file types policy can be added in the same profile.

Important Notes

• Disabling MOTW reduces a security control. Only apply this fix on managed devices where SharePoint and email content are already filtered by Defender for Office 365, Defender for Endpoint, or an equivalent layered control. Do not deploy globally to BYOD or unmanaged endpoints.
• Adding SharePoint domains to Internet Options > Security > Trusted Sites (for example https://*.sharepoint.com) helps in some cases but does not always stop OneDrive sync from writing the MOTW. The Attachment Manager policy is the reliable fix.
• If users still see the warning after the policy applies, check that Adobe Reader is not configured with Protected View set to Files from potentially unsafe locations. Edit Edit > Preferences > Security (Enhanced) in Adobe Reader.
• This setting only affects files downloaded after it is applied. To clear MOTW from existing files, run Get-ChildItem -Recurse | Unblock-File in PowerShell against the affected folder.

Related Resources

• Fix: Cannot Open HEIC Files on Windows (iPhone Photos)
• Outlook Error When Opening PDF Attachments: The Memory Could Not Be Read
• Fix: “This file can’t be previewed with this browser” in Xero
• Fix: “Action Blocked” When Opening Email Attachments in Outlook (ASR)
• Fix: Error Opening PDF Attachments in Outlook (Memory Could Not Be Read)
• Microsoft Learn: Attachment Manager security policy settings
• Microsoft Learn: Zone elevation and Mark of the Web
• Adobe: Protected View in Acrobat and Reader

If you need help rolling out Attachment Manager and PDF preview policies across your fleet, get in touch with our team.