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.

I recently read Contact(the book by Carl Sagan, still need to watch the movie), which features a tech billionaire who built his wealth doing exactly that. He developed a chip that could block TV commercials, and later one to filter televangelists as well.

For a book that was published in the 80s and set in the late 90s, it's prescient in a few very specific ways. We weren't exactly communicating by Portable Telefax in 1999, but adblockers were not far away either.

This guy went in expecting wacky on-ice hijinks and pranks, instead he got a heartfelt and weirdly wholesome comedy about a genuinely kind man trying to use his talent for violence to make something of himself and help his friends and teammates, often at great expense to his own health.

Herbert West just can't catch a break.

Somebody dress up like a wizard and go ponder this thing.