let's try like this

Makefile

@@ -3,7 +3,7 @@ OPTIONS += --filter=pandoc-numbering
 OPTIONS += --filter=pandoc-crossref
 
 PDFOPTIONS = --highlight-style kate
-PDFOPTIONS += --pdf-engine xelatex 
+PDFOPTIONS += --pdf-engine pdflatex
 PDFOPTIONS += --number-sections
 PDFOPTIONS += --template=./default.latex
 
@@ -20,7 +20,7 @@ SOURCES := $(filter-out 08_notes.md, $(SOURCES))
 all:  cours.pdf cours.html
 
 # %.tex: %.md
-# 	pandoc -s $(OPTIONS) $(PDFOPTIONS) -o $@ $<
+# 	pandoc -s $(OPTIONS) $(PDFOPTIONS) -o $@ $< --metadata-file metadata.yaml
 
 cours.pdf: $(CLASS_SOURCES)
 	pandoc -s $(OPTIONS) $(PDFOPTIONS) -o $@ $^ --metadata-file metadata.yaml