Wow, I really appreciate all of your responses!
Sorry I took so long to reply but my life has been very hectic!
I've been doing more research and it seems like Xpdf or Sumatra are the two best options for privacy and security because they're open source, however, it says here that Xpdf is closed source???
https://www.xpdfreader.com/opensource.html
I'm leaning towards Sumatra more now...