refactor makefile

Makefile

@@ -11,9 +11,9 @@ pdfs = $(patsubst %.md, %.pdf, $(markdown))
 $(pdfs): %.pdf: %.md
 	pandoc -t beamer -V theme:Warsaw -i $< -o $@
 
-.PHONY: show
-show: $(pdfs) ## Generate all pdfs (try `make -j 4 pdfs`)
+.PHONY: pdfs
+pdfs: $(pdfs) ## Generate all pdfs (try `make -j pdfs`)
 
 .PHONY: clean
 clean: ## Delete all pdfs
-	find . -name "*.pdf" -exec rm '{}' ';'
+	$(RM) $(pdfs)