Pdf: Nautilus Extension does not work?

hello,

i just installed mat2 and the nautilus extension on my debian stretch. i used this manual: https://0xacab.org/jvoisin/mat2/blob/master/INSTALL.md#debian

it works fine for nearly all formats, but not for pdf. if i open the context-menue of a pdf there is no "clean metadata".

do you know this problem?

greetings, nigra