All data that is stored / created by Site Report Pro such as Projects and PDF Reports are stored within the App and will therefore be backed up by your chosen iOS backup strategy.
Apple provides two backup strategies: iTunes and iCloud.
When restoring a device from a backup, the Site Report Pro app and its data will be restored also.
More information about how to backup your device can be found on the Apple website here: https://support.apple.com/en-gb/HT203977
Site Report Pro also offers the capability to export your projects manually to another location (e.g. a drop box account).
To learn how to export a project from Site Report Pro view the Export Project article.