Now I’m covinced iBooks hates me

I have plenty of space on my phone, yet every time I download a file into my iBooks app, it gets sent to the cloud and disappears from my device. I’ve tried turning off iCloud synchronisation, but that just means that my books don’t even show up on my device. If I’m not actively reading something, it just gets offloaded to the cloud and deletes itself from my device. It’s incredibly annoying, especially since I don’t have constant Wi-Fi access and want to read my books outside my home. How can I make it stop? I just want all of my files to stay on my devices