Office keeps an offline copy of every cloud document you open

Office on Windows is a cloud client that works offline, and the way it manages that is to keep a local copy of everything you touch. That is a good design with a storage consequence nobody mentions.

Where it is

Get-ChildItem "$env:LOCALAPPDATA\Microsoft\Office","$env:LOCALAPPDATA\Microsoft\OneNote","$env:APPDATA\Microsoft\Office" -EA SilentlyContinue |
  ForEach-Object {
    $s = (Get-ChildItem $_.FullName -Recurse -Force -File -EA SilentlyContinue | Measure-Object Length -Sum).Sum
    '{0,8:N0} MB  {1}' -f ($s/1MB), $_.FullName
  }

OneNote is the one that grows

OneNote keeps a complete local copy of every notebook you have ever opened, including images and attachments, and nothing removes one when you stop using it. Closing a notebook removes its local copy and it reopens from your account when needed, which is the correct fix.

Clearing the document cache

Office has its own control for this, in an app's options under Save, where the cache can be emptied. Use it rather than deleting folders, because a cached file with unsaved changes exists only there. Check the upload status first: a document that has not finished uploading has no copy anywhere else.

AutoRecovery files

Get-ChildItem "$env:LOCALAPPDATA\Microsoft\Office\UnsavedFiles" -EA SilentlyContinue |
  Sort-Object LastWriteTime | Select-Object LastWriteTime, Name

These belong to documents that were not closed properly. Anything older than a few weeks is from a session long finished, and is safe once you have confirmed the real document is where you expect.

The mail side

On a machine with Outlook, the OST file is usually larger than everything on this page put together, and it is worth checking on the same pass.