It is unlikely that you can convert pls to mp3, considering .pls file is a playlist, hence not actual audio file. a playlist is just a list of audio files that should be played in certain order. At best, you can open the playlist with a text editor and check the locations of the actual media files.
Open PLS file Open MP3 fileA music library management program - renamed to Helium (version 12) and replaced in 2016