Some online reader are very good websites to read manga online, but you can't download the archive or you need to be registered. If you have low-speed internet or you want to archive your manga, you can use MangaDLR.
MangaDLR is a manga downloader and reader that helps you to download your manga from many online readers and save them in zip archives so you can read them offline. You can read them with its viewer or another image/picture viewer app without an internet connection.
​Changelog:
Settings screen UI fixed.
Online search: search button on the keyboard.
Bookmarks: option to add one series in the app shortcuts (android 7.1+) or to pin a shortcut (android 8.0+).
Series screen: copy the name in the clipboard.
Fullcomic removed.
Fix some bugs.
Download: MangaDLR 6.8.0