How do you get PDFs to open in separate tabs, as opposed to separate windows?

I'm having an interesting issue. I've read that if you desire to open new PDF's as tabs, you are supposed to click the box entitled "Open documents as new tabs...", but it doesn't seem to show up under Basic Tools. If you'll look below, you'll see that the option doesn't even show up.

Does anyone have a solution to this? I've also checked to see if I'm operating on the latest version and it shows to be.