do not clean the PDFs

.gitignore

@@ -2,4 +2,3 @@
 *.fdb_latexmk
 *.fls
 *.log
-*.pdf

Makefile

@@ -6,4 +6,3 @@ clean:
 	find -type f -name *.fdb_latexmk -exec rm {} \;
 	find -type f -name *.fls -exec rm {} \;
 	find -type f -name *.log -exec rm {} \;
-	find -type f -name *.pdf -exec rm {} \;