Share your experience!
Did you try the included OfficeSuite?
Do you get any error messages?
I mean when you try to open a PDF file, do any error messages appear on your screen?
And you get this message with ALL pdf documents? Try downloading a trusted pdf document like Z1 user guide and see if it gives the same message:
http://www-support-downloads.sonymobile.com/c6902/userguide_EN_C6902-C6903-C6906_6_Android5.0..pdf