Why is ‘synced content’ taking up so much storage on my iPhone?

Since iOS 17 was released my iPhone and iPad have ‘synced content’ that is using up all my storage.

I use a pdf reader app called ‘GoodReader’ that allows me to store items in iCloud. The whole purpose of cloud storage is so I can free up storage on my devices, and access content in the cloud when I need it.

Now though, ‘synced content’ is taking up 350gb on my iPhone, and 80gb on my iPad.

Apple support have told me to de-select the apps that are using the storage in iCloud!!!

If I do that though, I can no longer access content in iCloud as the iCloud folder has gone!!!

I still don’t understand why I pay £8.99 a month for 2tb of iCloud storage, only to find that all that data is being synced onto my devices! Pointless!!