[RhodaLs]

You can very easily use your iPad as a reader for arbitrarily downloaded ebooks. Go to Gutenberg or Archive.org or the pirate site of your choice. Select a book. Select the format. Epubs and PDFs have the option of being sent to iBooks. Mobi can be opened with a compatible reader app if you have one installed. And the file can be saved to Dropbox or whatever file service you have installed. With the latest version of iOS (IpadOS) you can save to your Downloads folder or even to a connected storage device, or to a file share on your local network. You can also load books via Dropbox, local network, or connected storage.

I do this all the time. For me, it's convenient to use Dropbox as the intermediary, but there's plenty of other ways too.