This is not possible with PRTG itself.
You can compute the downtime for a particular time frame when you use the API to export historic data. However, it is not feasible to do this for every sensor, as this API call generates high load on the system.
The best option for any custom reporting task not supported directly by PRTG is to use the Data Extractor: https://www.paessler.com/tools/dataextractor
This application exports the historic data to an SQL database, allowing to use PRTG data with reporting suites or custom scripts to generate any kind of report.
Of course, data extraction also puts load on the system, but if you do it for example once per week, in the night, it is still a viable option.