If you are encountering difficulty retrieving your documents from your portal, an email, or the public archive on your Apple device(s), you may need to disable the pop-up blocker. Apple mobile devices default to pop-up blocker ON.
To change the settings on your device:
Settings-> Safari-> Block Pop-ups-> The switch needs to be OFF
After making this change, return to the link and attempt to download your documents again. Your Apple device will default to opening the item in iBooks unless otherwise specified.