Friends,
OOTB Audit Reports are Content Type Specific.
To Play with:
1. Activate Reporting Feature (Site Collection Feature listed OOTB in All Site Settings).
2. Go to Document Library > Settings > Information Management Policy Settings > Click Content Type > Define Policy > Enable Auditing with desired options > Ok.
3. Go to Site Collection Audit Settings > Check desired options.
4. Audit Log Reports.
It has a limitation of 2000 documents per content type, won't work for more than mentioned & will flash very mech known: Unknown Error.