When you tap on a filename in the mobile app we attempt to display the file in the mobile web browser. If an “Unable to Read Document” error is displayed it could be because the file in question is password protected.
If you have the Adobe Acrobat app installed, tap the “Open in…” link at the top-left of the page and select that app to view the file. It will prompt you to enter the password for the file. If you have Apple’s Pages app installed you can also tap the “Open in “Pages”” link at the top-right. Pages will also prompt you for a password.