add invoice pdf

lib/helpers/cache_mgr.dart

@@ -0,0 +1,12 @@
+import 'package:flutter_cache_manager/flutter_cache_manager.dart';
+
+class PdfCacheMgr {
+  static const key = 'pdfs';
+  static CacheManager pdfs = CacheManager(
+    Config(
+      key,
+      stalePeriod: const Duration(days: 7),
+      maxNrOfCacheObjects: 20,
+    ),
+  );
+}