There are password protected page1.pdf, page2.pdf, etc... under mnt/sdcard/com.zinio.mobile.android.reader/9380198313/123123231/32123213 etc... Can I delete them?
If you don't need them anymore, then you should be able to delete them. I'm not familiar with Zinio, but is there a way to safely delete the magazines within the application? I would try that first.
Deleting these files may cause app instability if it is expecting them to be there (when they aren't). Try moving them somewhere else (don't delete them just yet) and see how the application behaves with them "missing/moved".