In K2 Five, we added a new header for security reasons (X-Content-Type-Options=NoSniff). This caused the "web-view/Browser" to not be able to display the file in K2 mobile app, because the content type that gets returned in the response is wrong.
It is a known issue that will be addressed on the SmartForms end (not the mobile app). The workaround is to comment out this line in the Smartforms Runtime/Designer web.config
1. Backup the web.config for Smartforms Designer/Runtime web.config. "C:\Program Files\K2\K2 smartforms Runtime\Web.config"
2. Edit the web.config file and comment out this line <add name="X-Content-Type-Options" value="NoSniff"/>
3. Restart K2 service and test it again.