Any reports available on Document Library usage?

Is reporting feature turned on for your Site? You should see an option Generate file plan report under Library Settings which should give you quick glance of details of files and folders within your library

https://www.c-sharpcorner.com/UploadFile/644e54/generate-file-plan-report-in-sharepoint-2013-and-office-365/

Also under site collection administration section there are audit log reports available which you can filter to a specific library and analyze information regarding who has done what with files in the library

https://support.office.com/en-us/article/view-audit-log-reports-b37c5869-1b47-4a82-a30d-ea20070fe527

 You can also get more detailed report regarding activity in a document library under security center audit log search reports.

https://docs.microsoft.com/en-us/office365/securitycompliance/search-the-audit-log-in-security-and-compliance