How to delete all my photos at once on my iPad Pro, without messing other documents or data

Deleting all of your photos should have no bearing on documents and data, even if you use iCloud Photos.

If the photos that are on your iPad were synced with iTunes on your computer, you have to unsync them using iTunes again.

If the photos were from screenshots, saved from the web or emails, or were photos that you took on the device, you can select them and use the trash icon to remove them. The photos can be batch selected selecting one photo at the bottom left or right corner, then swipe your finger to the left or right and then up the screen all the way to the top to select all photos. Then after they are all selected, tap the trash icon.

If you are using iCloud Photos, you can turn it off to remove the photos, but before you turn off iCloud Photos, you might want to make a copy of your photos and videos.