I get unused file extension for PDF_Profile when I run Registry on CCleaner Free (v4.19.4867 (64 bit) ). This file extension is used in Serif's Page Plus 8 (32 bit) for publishing profiles. (I'm using the 32 bit version of PP8 because the 64 bit version doesn't recognize certain databases such as excel that I use.)
If that extension is for a software you have installed you can stop the registry cleaner from detecting it by:
Right-click -> Exclude