To see the files, your phone needs to be rooted.
Opera team has never ever bothered adding the option to Opera Mini to save webpages to SD card ever since Opera Mini was first released for Android.
So when we uninstall Opera Mini or move to another phone, the saved webpages are gone forever. In my opinion, the Saved Web pages feature in Opera Mini for Android should go, unless they can offer the option to save them to SD card.
Only the Java version of Opera Mini has, and presently enjoys this feature.