Without Collabora or the Documents app installed, is there a preview option for Office files? Like the PDF viewer ideally. It’s a little frustrating having the files immediately download when all I want to do is quickly check the file in the browser.
I have no documents-related apps installed, simply because the documents app isn’t supported on 11.x and Collabora is far too overkill for my needs.
I saw that (and considered hijacking it), it’s looking to be thumbnails rather than a PDF viewer-type app I’m looking for.
Okay, comment edited. 
Regarding the Documents app, Nextcloud wrote on Google+:
Everything in Nextcloud happens either because a volunteer feels like doing it or the company decides to pay for it - there is only so much the business can do, and it is mostly driven by customer demand. Sadly for users who like the Documents app, there is no customer demand for it so we have put it as low priority 
Yeah I saw that too. In the interim I need a viewer that won’t try to edit the documents, just display them for me without having to convert to PDF (as has been suggested a few times)
With the Documents app v0.13.1 I can open/view simple .odt documents in the Files app, but not within the Documents app.