Check .. download directive for PDF pipeline

[mmcky]

From QuantEcon.lectures

The PDF file ignores the :download: directive. For now a temporary solution is to specify a latex only alternative that links to our hosted pdf file that lives in _static/_downloads and is generated by the html compilation step.

.. only:: html

    We begin by discussing an example of a Bewley model due to :download:`Rao Aiyagari <{{ pdfs_dir }}/aiyagari_obit.pdf>`

.. only:: latex

    We begin by discussing an example of a Bewley model due to `Rao Aiyagari <{{ download_web_dir }}/aiyagari_obit.pdf>`__