To identify which apps are taking up the most space simply goto  Settings > General > Storage & iCloud Usage > Manage Storage.

If PIM is taking up too much space then in a recent version of PIM (v3.2.4) we have made some improvements in the way photos are stored. PIM now stores them at a more acceptable resolution without taking up too much space on the iPad. 

We have also included the ability for all existing photos to be upgraded to the correct and more optimal resolution. If you go to Settings > Advanced and tap on Optimize Photos, this will retrospectively optimize the photos and resize them accordingly.  See screenshots below to help you find this function.


Note that this may take a while (depending on the number of photos affected) and it’s probably worthwhile to ensure the PIM app has been backed up recently. Whilst it is performing the upgrade it is also worthwhile changing the iPad screensaver so the PIM screen remains active and the iPad doesn’t switch into sleep mode. Once you perform that action you will notice the size of PIM on your iPad will drop dramatically and this can be checked under Settings > General > Storage & iCloud Usage > Manage Storage. If the iPad's free space is still too low it might be worth investigating other apps if they are taking up too much space and free space accordingly in those apps.