They sure are; please check <PRTG Data Directory>\Logs\webserver\webserver.log. The others are rotated by date. The data directory itself is disclosed in the PRTG Administration Tool, in the Core Server tab. If you haven't changed it, it's C:\ProgramData\Paessler\PRTG Network Monitor\.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.