Overview & background
Fieldwire has the option to permanently store files (Plans and Files, photos are not included yet) uploaded to Fieldwire to your devices' internal storage. This feature is tied to your profile so, like smart synchronization, it will apply across all enabled projects on your device. The purpose of this is to ensure that users on mega projects can access all of their files when offline.
When this option is disabled, files are stored in your device's cache (instant memory). Disabling Permanent File storage reduces the amount of device storage Fieldwire uses, but it may mean you cannot access old project files when offline.
Note: Permanent File storage is Device Specific, this means that it can be enabled on one device, but disabled on another.
Example: Fieldwire is installed on your iPad and iPhone so you can have Permanent File Storage enabled on your iPad, and have it disabled on your iPhone.
How to enable permanent file storage
Step 1:
To enable permanent file storage, first select the 'Profile' option from the project dashboard. 
Step 2:
Once under 'Profile', select the option to 'Store files permanently on device'.
Note: This feature is disabled by default to preserve storage space on your device. If this option is enabled, files that are synced or viewed are permanently saved to your device. If this feature is not enabled, then the files will still be available in your device's cache.
Tip: If you choose to disable permanent storage at some point after it is enabled, new files are saved to the cache rather than the internal memory. To remove the files that have been already saved to the internal memory, choose to disable the project.
More Information
These articles provide more information into synchronization and managing your iOS devices' storage space.