Oops! Resetting the phone fixed this.
I don't think I had installed any apps at all, before experiencing this. So, maybe the OS was updated? I don't know.
The phone was purchased from Verizon.
After resetting the phone, when I tapped, it prompted me to download an App to view the PDF. It was an excel file from a different version of excel that I tried after resetting. So, I don't know if not prompting for an App is related to tap not working for an excel file before.