Why are kiCAD Net Labels not searchable when converted to pdf?
This is a feature in the next version of KiCad (v5).  See https://bugs.launchpad.net/kicad/+bug/1710454 for details.

Quoting from link:


  jean-pierre charras (jp-charras) wrote on 2017-08-15:
  Commit 0999b5cb77c9a1b44e7e194071c2acb7e33ffb1d should work in PDF files.
  (search texts are enabled in Eeschema and Pcbnew).
  
  Please try it (especially with texts using non ASCII8 chars)


If you want to try out the v5 RC, you can download a nightly build from 


http://downloads.kicad-pcb.org/windows/nightly/ (Windows)
http://downloads.kicad-pcb.org/osx/nightly/ (Mac)
http://ppa.launchpad.net/js-reynaud/kicad-5/ubuntu/ (Ubuntu)


Warning: While KiCad 5 is currently (5/14/18) at Release Candidate, not all known bugs are fixed and it is possible that you will experience crashes and/or data loss.  Using Version 5 on your v4 schematics and layouts will require you to convert them to a new, non-backwards-compatible format.  Be sure to back up your data before trying and keep backups during your work.  And remember to report any bugs so they get fixed!