File Explorer not previewing pdf files since 25H2 update

Paul Nicholson 560 Reputation points
2025-10-15T08:07:37.1033333+00:00

Since installing the 25H2 update to Windows 11, the preview for pdf files is not working.

I just get the following 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"

I have tried everything to restore the preview without success.

Any help would be much appreciated!

Windows for home | Windows 11 | Files, folders, and storage
Answer recommended by moderator
Vivekanandan Tiwari 210 Reputation points
2025-10-20T03:18:02.11+00:00

Here is a simple solution that worked for me -

in the file explorer, just disable this check box as shown in the image, this simply worked for me

(This happened on windows 10 as well!)
Screenshot 2025-10-20 084655

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  1. Mr K 5 Reputation points
    2025-11-13T14:29:46.7166667+00:00

    The culprit is Windows update KB5066835 - if you check the update notes on the windows website its the last on the list of features-->

    https://support.microsoft.com/en-us/topic/october-14-2025-kb5066835-os-builds-26200-6899-and-26100-6899-1db237d8-9f3b-4218-9515-3e0a32729685

    I managed to get windows support chat to give me a code to disable it, but it seemed to only work for files received up to that date, nothing received since. Trying to find a long term fix, any suggestions would be greatly appreciated! This update has completely killed my workflow!

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  2. Peter 5 Reputation points
    2025-11-11T12:33:21.41+00:00

    Just to add another variation to the answers here:
    I open the specific folder (in explorer) I want to unblock and "Open in Terminal" (PowerShell) then use the below:

    Get-ChildItem -Path ".\" -Recurse -Filter "*.pdf" | Unblock-File

    I like these PowerShell Unblock-File options over the Internet Security and Registry options as I don't want to just open a blanket security hole that this update closed.

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  3. A 5 Reputation points
    2025-11-10T00:10:45.7033333+00:00

    Happend to us as well, using Server 2025 and Windows 11.
    No fixes listed work.
    Has impact to business workflow can't uninstall update.
    Do not upgrade, always wait we normally do and know better!
    Hope there is a fix released urgently.

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  4. GenC 25 Reputation points
    2025-11-09T18:23:51.2433333+00:00

    For those who have many files affected by this (such as extensive OneDrive users, or many files printed from Edge) but do not want to blindly unblock every file, here is a PowerShell script that can be run to unblock all PDF files in a directory that you've reviewed. Save as a .PS1 script file in a directory that is in your environment %PATH% (search "environment variables" in Settings) with a name such as UnblockPdfsInDir. Then when viewing a directory in Windows Explorer, you can right-click on the folder, choose "Open in Terminal" and just type the script file name in the terminal window that opens. The script will locate and unblock all the PDF files in that folder.

    I tried several of the other solutions in this thread, but encountered a number of errors, particularly with filenames that included characters interpreted by PowerShell as wildcards, so the script has substantial error-checking to catch those. It also reports on the number of PDF files found, how many were unblocked, how many were already unblocked, and any unhandled errors.

    This script unblocks only PDF files, but can be modified for any other file type, or multiple types. 

    <# Unblocks [PDF] files in current directory #>
Write-Host
$Dir      = Get-Location
$ErrCnt   = 0
$Type     = "pdf"
$FileCnt  = 0
$BlockCnt = 0
$NoBlock  = 0
$Filter   = "*." + $Type
Write-Host "Starting unblock files in directory:"
Write-Host $Dir -ForegroundColor Blue
Get-ChildItem -Path $Dir -Filter $Filter |
    ForEach-Object {
        $FileCnt += 1
        $Stream = ""    ## (clear $Stream variable so that empty if Get-Content fails in try block ##
        try {
                ## (use -LiteralPath instead of -Path to avoid error with some filenames interpreted as wildcards) ##
                ## (use -ErrorAction Stop to promote errors to terminating so that catch blocks will work) ##
                $Stream  =  Get-Content -LiteralPath $_ -Stream "Zone.Identifier" -ErrorAction Stop
        }
        catch [System.IO.FileNotFoundException] {
                IF ($Error[0].Exception.Message -like "Could not open the alternate data stream*") {
                    $NoBlock += 1
                } else {
                    $ErrCnt += 1
                    Write-Host "Err ${ErrCnt}: Unrecognized FileNotFoundException (message below):" -ForegroundColor DarkYellow
                    Write-Host $Error[0].Exception.Message -ForegroundColor Blue
                }
        }
        catch {
                $FQID = $Error[0].FullyQualifiedErrorId
                $ErrCnt += 1
                Write-Host "Err ${ErrCnt}: Unexpected error $FQID (details below):" -ForegroundColor DarkYellow
                Write-Host $Error[0].Exception.GetType().FullName -ForegroundColor Red
                Write-Host $_.Exception.Message -ForegroundColor Red
        }
        
        If ($Stream -contains "ZoneId=3") {
          $BlockCnt += 1
          Unblock-File -LiteralPath $_      ## (use -LiteralPath instead of -Path to avoid error with some filenames interpreted as wildcards) ##
        }
    }
$TypeUpper = $Type.ToUpper()
Write-Host "$FileCnt $TypeUpper files in directory; $BlockCnt files unblocked ($NoBlock already unblocked, $ErrCnt errors)" -ForegroundColor Green
Write-Host "File unblock completed."
Write-Host

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