In testing my answer to this question, I initially wrote the file
%& eplain % code to answer question here \bye
and compiled with
pdftex e. Much to my surprise, the penultimate line of output was
Output written on e.dvi (1 page, 352 bytes).
If I change the first line to
\input eplain or add the command line argument
-output-format=pdf, then I get a pdf.
This same behavior can be observed by by starting a LaTeX file with
%& latex and compiling with either
pdftex so it's unrelated to