You can transfer your own files to practically every ereader out there. I have no knowledge about iPads, but it's as easy as connect your reader to your computer with a USB cable and drag/drop your files to the reader. Where exactly you drop the files depends on the reader (for example, on a Kindle you'll have to drop them to the folder named "documents"). Also the files must be in the right format for the device (Kindles don't read epubs, so the file must be mobi or azw3 - the latter is the better, more modern format). Mobi support on a Kobo is rather poor, so the file should be epub or kepub (kepub is Kobo's native format). For a Nook it's epub. Converting from one format to another is easy and free - you can use various online converters or download and install Calibre (
https://calibre-ebook.com/).