You cannot download the web browser version of Backstory for offline reading.
On a tablet, you can do this on a per issue basis.
Once inside an issue you have bought individually, or an issue you have access to as a subscriber – use the down arrow icon to download the issue onto your tablet.
After you’ve read the issue, you can then archive it and free up space on your tablet.
NOTE: Backstory issues are commonly 200 – 500 mb.