Based on existing reports in Enterprise, Reports, Information Publisher Reports you can define your on reports if you use the Create Like option.
However save as PDF is not an option but you can Print the reports as PDF.
Or install BI Publisher.
Is there any option to export the list of all databases monitored through OEM 12c?
I need some more information like Total number of hosts, all host names, CPU Sockets, CPU Core, Operating system details of each hosts.
It would be helpful if I can export the details in xls or html or pdf format.
Enterprise -> Configuration -> Inventory and Usage. can export to excel from there and query various ways. Part of LifeCycle Management.