Download RSS feed as xml file from Sharepoint Online using PowerShell

Our company sharepoint (Office 365) contains also several RSS feeds.
How it is possible to download xml file out of this feed (website) using PowerShell?
I can authenticate with sharepoint using CSOM but do not what to do next.
As service user is not administrator I cannot use "SPOService".
This script works OK for standard website, but not for Sharepoint.
$doc = New-Object System.Xml.XmlDocument
I am getting this error when using for company Sharepoint:
"The remote server returned an error: (403) Forbidden."
Thanks for your time considering this question.

Hi Scott,
thanks for advice.
Combination of WebClient and Sharepoint Credentials is working OK:
$client = New-Object System.Net.WebClient 
$client.Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($User, $SecurePassword)
$client.Headers.Add("X-FORMS_BASED_AUTH_ACCEPTED", "f")
$client.DownloadFile($SiteURL, $file)