The P in PNaCl is Portable.

Pepper was a pun on Native Client (since NaCl = salt). Pepper Plugin API (PPAPI) was Google's more secure version of NPAPI (Netscape Plugin API). Flash Player was essentially the only thing using NPAPI/PPAPI by the end of its life.

The most common plugins were Flash, Silverlight, Adobe Reader, and the Java applet plugin, and I think all of those were in mildly common use when plugins were on their last legs.