reader_sl.exe is a process associated the with the Adobe Reader. It is used to decrease the load time for the reader when a PDF document is selected.
This is a non-essential process so disabling or enabling it is down to user preference.
Non-system processes like reader_sl.exe originate from software you installed on your system. Since most applications store data in your system's registry, it is likely that over time your registry suffers fragmentation and accumulates invalid entries which can affect your PC's performance. It is recommended that you check your registry to identify slowdown issues