question

try with the below client code:

 string filename = "YourFileName.extension";
 Response.AppendHeader("Content-Disposition","attachment; filename="+ filename  +"");

https://www.codeproject.com/Questions/508368/Topluschangeplusfileplusnameplusafterplusdownloadi

Thanks & Regards,

Gotcha, So you want to download files from the library to local system and then rename the files in a Library.

Below script will download all files from library to your local system:

 #Config Variables
 $SiteURL = "https://crescenttech.sharepoint.com/sites/marketing"
 $FileRelativeURL = "/sites/Marketing/Shared Documents/Discloser Asia.docx"
 $DownloadPath ="C:\Temp"
     
 #Get Credentials to connect
 $Cred = Get-Credential
     
 Try {
     #Connect to PNP Online
     Connect-PnPOnline -Url $SiteURL -Credentials $Cred
         
     #powershell download file from sharepoint online
     Get-PnPFile -Url $FileRelativeURL -Path $DownloadPath -AsFile
 }
 catch {
     write-host "Error: $($_.Exception.Message)" -foregroundcolor Red
 }

This below script will rename all files from Library.

 #Config Variables
 $SiteURL = "https://crescent.sharepoint.com/sites/marketing"
 $LibraryName = "Documents"
      
 #Connect to PnP Online
 Connect-PnPOnline -Url $SiteURL -Credentials (Get-Credential)
     
 #Get All Files from the document library
 $Files = Get-PnPListItem -List $LibraryName -PageSize 1000 | Where {$_["FileLeafRef"] -like "*.*"}
      
 #Loop through each File
 Write-host -f Green "Number of Files Found:"$Files.Count
 ForEach($File in $Files)
 { 
     Write-Host ("Renaming File '{0}' at {1}" -f $File["FileLeafRef"], $File["FileRef"])
     #Rename File
     Rename-PnPFile -ServerRelativeUrl $File["FileRef"] -TargetFileName "Old_$($File['FileLeafRef'])" -OverwriteIfAlreadyExists -Force
 }

Reference:
https://www.sharepointdiary.com/2016/09/sharepoint-online-download-file-from-library-using-powershell.html
https://www.sharepointdiary.com/2017/10/sharepoint-online-rename-files-in-document-library-using-powershell.html

Thanks & Regards,

If you could accept not to use WebClient, you can use PowerShell CSOM to download the file and rename the file in one method.

 Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.dll"
 Add-Type -Path "C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.Runtime.dll"
  
 Function DownloadAndRename-FileFromLibrary()
 { 
     param
     (
         [Parameter(Mandatory=$true)] [string] $SiteURL,
         [Parameter(Mandatory=$true)] [string] $SourceFile,
         [Parameter(Mandatory=$true)] [string] $TargetFile,
         [Parameter(Mandatory=$true)] [string] $NewFileUrl
     )
  
     Try {
         #Setup Credentials to connect
         $username = "amos@contoso.onmicrosoft.com"
         $password = "password"
         $Cred = New-Object -TypeName System.Management.Automation.PSCredential -argumentlist $userName, $(convertto-securestring $Password -asplaintext -force)
         $Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($Cred.Username, $Cred.Password)
         #if you are using SharePoint On premise,just delete the above Credentials script
         #Setup the context
         $Ctx = New-Object Microsoft.SharePoint.Client.ClientContext($SiteURL)
         $Ctx.Credentials = $Credentials

         $FileInfo = [Microsoft.SharePoint.Client.File]::OpenBinaryDirect($Ctx,$SourceFile)
         $WriteStream = [System.IO.File]::Open($TargetFile,[System.IO.FileMode]::Create)
         $FileInfo.Stream.CopyTo($WriteStream)
         $WriteStream.Close()     
         Write-host -f Green "File '$SourceFile' Downloaded to '$TargetFile' Successfully!" $_.Exception.Message
         $File = $Ctx.Web.GetFileByServerRelativeUrl($SourceFile)
         $File.MoveTo($NewFileURL, [Microsoft.SharePoint.Client.MoveOperations]::Overwrite)
         $Ctx.ExecuteQuery()     
         write-host -f Green "File Renamed successfully!"
   }
     Catch {
         write-host -f Red "Error Downloading File!" $_.Exception.Message
     }
 }
  
 #Set parameter values
 $SiteURL="https://contoso.sharepoint.com/sites/dev/"
 $SourceFile="/sites/dev/Shared Documents/1.pdf"  #Relative URL 
 $TargetFile="C:\Temp\1.pdf"
 $NewFileURL="/sites/dev/Shared Documents/2.pdf"
 #Call the function to download file 
 DownloadAndRename-FileFromLibrary -SiteURL $SiteURL -SourceFile $SourceFile -TargetFile $TargetFile -NewFileUrl $NewFileURL

Reference:
https://www.sharepointdiary.com/2016/09/sharepoint-online-download-file-from-library-using-powershell.html
https://www.sharepointdiary.com/2017/10/sharepoint-online-rename-files-in-document-library-using-powershell.html
PNP powershell suggested by sharatha is also another solution.
PNP powershell installation

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