/usr/share/texlive/texmf-dist/tex/plain/hyplain/hypdfplain.ini is in texlive-plain-extra 2014.20141024-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 | %%% This is part of the hyplain package by Enrico Gregorio
%%% You are allowed to do anything you want with this as long
%%% as you cite the source and change name to the file
%%%
%%% The purpose of this file is to provide an initialization
%%% for pdfTeX
\pdfoutput=1
\input hyplain
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