Please does resetting the PRAM erase files on the iMac

No. Resetting the PRAM (Parameter RAM) simply clears hardware registers of possible corruption. It has nothing to do with the file system. However, it is always a good practice to make a full backup before doing any kind of troubleshooting... JUST IN CASE.

No, it does not. Please read https://support.apple.com/en-us/HT204063

No' to understand what NVRAM does, see > Reset NVRAM or PRAM on your Mac - Apple Support