Downloading Manga

I'm gonna be on a plane soon and I wanna download manga offline (for free). I used to be able to use the app 'tachimanga' and I could download mangago stories through that app. I tried redownloading the app, but I don't really get what's going on with it. The 'extensions' thing don't really make sense to me even through the FAQs. When I used it before I could access and download everything through there not get it from somewhere else and somehow access it on the app idk.

Anyone know what app or site I can use to effectively download free manga?