Is SystemCleaner agent running? It should ideally run daily and trim off the log contents > 30 days.
Pr_Log table --> Default table for Log- instances such as Log-SecurityAudit; stores instances of concrete classes derived from the Log- base class other than the classes specifically associated with other tables. I have also attached the document reference which will give you the descirption of all the tables of PRPC.
There wouldn't be any issue in deleting the log entries. You can use OOTB TrimLog activity that accepts Log class and day's threshold to trim entries older than the specified threshold.
ClearLog activity can also be used to clear off entire log entries. It accepts log class as a parameter.