Office Viewer app?

We have office 2021 licences (not 365) and we use them to edit offline.
Files are in NextCloud. Is there any free online office viewer available that would allow to view word and excel files without downloading them? So as PDF viewer (PDF viewer - Apps - App Store - Nextcloud) for example?
Only Office or Collabora office we would have to buy licences for 30 people (we are 30 in the office), right? – even if we only need to view the documents, not edit them.

Thank you for advice