How can I report on who has viewed a file in an Office 365 SharePoint library?

Solution 1:

You can enable audit logs for this. This can be enabled at site collection level.

Go to site settings--> Site collection administration-->Site collection audit settings-->Opening or downloading documents, viewing items in lists, or viewing item properties. You can take a look at the following article.

http://office.microsoft.com/en-us/sharepoint-server-help/view-audit-log-reports-HA102039795.aspx