When clicking on a pdf file with .pdf extension I get the following
error dialog:
The filename "Anlage_2_EW_2009_Gesamt_g.pdf" indicates that this file is
of type "pdf document". The contents of the file indicate that the file
is of type "PDF document". If you open this file, the file might present
a security risk to your system.

Do not open the file unless you created the file yourself, or received
the file from a trusted source. To open the file, rename the file to the
correct extension for "PDF document", then open the file normally.
Alternatively, use the Open With menu to choose a specific application
for the file.

Note that pdf files without pdf extension open just fine.