The bug was that trying to download the cooperator certificate or
another PDF in the interface brings an error 500. This was due to a
wrong usage of the function `get_pdf()` and also due to a error of
encoding when using `format()` python function with `{name}`. Instead I
use `"%s" % name` that works great.
It didn't use `get_pdf()` right because of an old version of mis-builder
that cause the methord `get_pdf()` to work weird. With an updated
version of mis-builder, `get_pdf()` can be used correctly.