Invoking
    emacs  /path/to/file.tex
followed by C-c C-f does work.

But doing
    emacs  /path/to
then opening file.tex from dired followed by C-c C-f does not work.

Apparently tex-mode.el is *not *sending the correct full path to pdftex; it
does *not* find the input file under many circumstances.

Guido