I'm sorry to hear you are having trouble with SmartOffice.
First, I need a bit more detail. Can you give us specifics as to your current environment:
amount of storage space (used/available)
Exact iOS version (9.x)
Exact SO2 version (2.x.x)
Next, I'd like to recommend that you close all applications, then power off the device, then restart the device, then try opening SO2 before opening any other applications. If you get any error messages, please take a screen shot for us to review.
PS - I just realized you said the PDF was attached to an email. After the reboot, you'll need to open your email app to choose SO2 from the "open with" choices.