[lint] FIX SC2006: use

pdf-optimize

@@ -136,11 +136,11 @@ ${bold}$output_path${normal}."
 
 function compute_compression
 {
-	input_size=`du "$input_path" | cut -f1`
-	output_size=`du "$output_path" | cut -f1`
-	percentage=`echo "scale=1;$output_size / $input_size * 100" | bc -l`
-	input_size_h=`du -h "$input_path" | cut -f1`
-	output_size_h=`du -h "$output_path" | cut -f1`
+	input_size=$(du "$input_path" | cut -f1)
+	output_size=$(du "$output_path" | cut -f1)
+	percentage=$(echo "scale=1;$output_size / $input_size * 100" | bc -l)
+	input_size_h=$(du -h "$input_path" | cut -f1)
+	output_size_h=$(du -h "$output_path" | cut -f1)
 
 	echo "Size reduced from ${bold}$input_size_h${normal} to \
 ${bold}$output_size_h${normal} [$percentage %]"