What document should I put on iCloud?

I'm not sure exactly what you mean.

If you mean that you want to free up storage space on an iPhone, usually photos and videos is the first thing to consider as they take up a lot of space, especially if you have many of them. To free up space that photos and videos use, turn on iCloud Photos and also turn on the Optimize Storage option for photos. See information in Set up and use iCloud Photos - Apple Support

Optimize Photos option allows your phone to automatically free up space by storing smaller space-saving version of photos on your phone if the phone is low on space. Also be aware that Cloud Photos is a syncing service. Any photo you delete from a device where iCloud Photos is turned on will also be deleted from iCloud and from any other device connected to iCloud Photos. You can potentially reduce the storage space required on a device by turning on Optimize Storage. See the following Apple article for more information: Set up and use iCloud Photos - Apple Support

Apple does not currently offer cloud-only storage for photos. If that is what you want or need, you may want to explore 3rd party options such as Google Photos or Dropbox.