Sonic PDF Creator for iOS app works in following way:
Find the file you want to convert, open it, press the Share button, and from Open In menu choose Sonic.
The conversion will automatically start.
However, to recieve converted result, all notifications for Sonic should be enabled.
You can check that in Settings app, go to Notification Center, and then scroll down to find Sonic, tap on it, and enable all notifications if they aren't already.