Reply To: Split long document into several smaller documents?

I would not drag pages into a new document; weird things can happen. Instead, I would delete pages 30-170 and then do a File > Save a Copy, then File > Revert (or perhaps even just Undo). Then delete 1-29 and 50-170 and save a copy, repeat, etc.

(You want to make sure you do a save as or save a copy after deleting in order to clean up the extra stuff, so it doesn't get stuck in the file, as per the Lose 20 Pounds in InDesign article Mike Rankin and I did for indesign magazine.)