macbook air 800 typed pages need external hard drive.

Not for that. Internet searches indicate that an 800 page Microsoft Word document on average is somewhere between 2 and 16MB which is tiny by today's storage standards.

Keep a copy of your 800 typed pages document in more than one place for safe keeping. On that external drive, on a Time Machine backup drive, and on your local Mac drive. An 800-page Pages document will definitely be larger than an equivalent number of pages generated by Microsoft Word, but not large enough that it would require a dedicated external drive for it alone.