While the names are very similar, there is a big difference between granting “Read” permissions or “View-Only” permissions in Office 365 SharePoint online.

Both provide view access to the files.
Only Read allows access to find the files using search.
Only Read allows access to download the files.

That’s it in a nutshell. So if your user can see a document, but not download it, you will need to grant the user’s account Read access to the library the document is in.