Why do some files not have previews?

When you open Documents, a preview thumbnail is displayed for documents that are stored locally, on Google Drive or ownCloud. Those stored in OneDrive show as missing (or blank) preview thumbnails.

If you download a OneDrive document to local storage through the web interface, a thumbnail will be generated.

The local copy of a document downloaded from an online account will lose its ability to be updated online. If you want to continue to edit it online, it is better not to download it.