Any document you run the code on will be split into ~1000 word segments. It's a once-off exercise, though and, if you need to make further splits, re-running the macro will simply regenerate the previous splits (subject to any edits you've made there).
For PC macro installation & usage instructions, see:
http://www.gmayor.com/installing_macro.htm
For Mac macro installation & usage instructions, see:
https://wordmvp.com/Mac/InstallMacro.html