If it is in iCloud the document will be saved to the Pages folder in your iCloud Drive, which in Files app on your iPad will be found by using the Browse tab, iCloud Drive, and scrolling down to Pages.
The document is taking the same amount of storage space in two places; one on your iPad and the other on iCloud. If you open the document for the first time on another device the iCloud copy is synced to this new device and exists in three places.
You can also sign into Pages in iCloud and work on the document from there in your browser, but in an environment which replicates the look of the Pages app on a device.