If you just want to read PDFs the built in Preview application is more than sufficient for that. I would not install the Adobe Reader unless there is a specific feature in it you require.
I recommend you install it. While the Preview app is more than sufficient for most PDFs, there are some that it doesn't display properly. Also, there are a few fillable forms that won't work with Preview, but will work with Adobe Reader. I keep Preview as the default app for PDFs, and only launch Adobe Reader in those situations where Preview doesn't do the job.