SharePoint Online - Upload document using PowerShell / CSOM

Please check the below modified script, which generates the byte array from the given URL and upload it to the SPO.

$wc = New-Object System.Net.WebClient
# update with correct path 
$ByteArray = $wc.downloaddata("http://downloads.microsoft.com/download/file.exe")

#Specify tenant admin and site URL and including DLL
$User = "XXXXX@XXXXXXX"
$Password = 'XXXXXX' | ConvertTo-SecureString -AsPlainText -Force
$SiteURL = "XXXXXXXXXXX"
$DocLibName = "Documents"
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"


#Bind to site collection
$Context = New-Object Microsoft.SharePoint.Client.ClientContext($SiteURL)
$Creds = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($User,$Password)
$Context.Credentials = $Creds

$List = $Context.Web.Lists.GetByTitle($DocLibName)
$Context.Load($List)
$Context.ExecuteQuery()

#Upload File
$s =[System.IO.MemoryStream]($ByteArray)
$FileCreationInfo = New-Object Microsoft.SharePoint.Client.FileCreationInformation
$FileCreationInfo.Overwrite = $true
$FileCreationInfo.ContentStream = $s
$FileCreationInfo.URL = $UniqueFileName
$Upload = $List.RootFolder.Files.Add($FileCreationInfo)
$Context.Load($Upload)
$Context.ExecuteQuery()