Originally Posted by ordx

I've also had issues with Reading List. It seems to work if there's absolutely no connection, i.e. in Airplane Mode without any WiFi enabled. But if it's simply a weak connection, Safari will try to reload the page unsuccessfully and lose whatever was cached previously. So you never know if the page will be available for offline reading.

Rather than try to gamble with Reading List, you can use a third-party app instead. I've used Pocket and there's also Instapaper which has similar functionality.