It’s not a device that Sagan goes into much detail about, aside from it being a new and less-than-reliable technology in the early parts of the story. I always imagined it as a laptop-sized, wireless fax machine using cellular networks to share data. Characters mostly use paper documents throughout the book, and while there are some sci-fi technologies like holographic displays that advance throughout the story, Sagan never describes anything like portable computers or smartphones. Even the internet(or its closest approximation) never goes beyond a rudimentary data-sharing network for astronomers, never open to the public.

A quick google search tells me EFax would probably work over that network, sending documents from a desktop straight to someone’s Portable Telefax like an email, so you’re not far off.