"Science Fiction is speculative fiction in which the author takes as his first postulate the real world as we know it, including all established facts and natural laws."
- Robert Heinlein

See also this usage from their 1981 novel Oath of Fealty.

"Sinclair wants to do some outside work while we're dirtside." He took out his pocket computer and wrote quickly with the attached stylus. "Let's see, we're about one and a half billion kilometers from New Scotland, that's uh, make it a hundred hours to turnover. About two hundred hours' travel time, plus what we spend on Dagda...

Apparently, they also included some sort of wireless link, because (elsewhere in the novel) it says that, when the officers were off duty, they "could always be reached on their pocket computers."

Another early mention of a small "pocket computer" or note-taking device with some mathematical functionality built-in is the calculator pad from Foundation by Isaac Asimov.

Asimov also mentioned a "pocket-computer" in his 1975 story Point of View.

Roger’s father looked as if he were thinking very hard. He took out his pocket-computer and tried some combinations on it. He tried some more combinations. Then he said, "You know, Roger, if I take what you said and turn it into Platt-integrals, it makes a kind of sense..."

As far as I know, the first pocket computer sold as such was the TRS-80 PC-1 in 1980. It weighed 6.0 oz., had 1.5 kilobytes of RAM, was programmable in BASIC and cost $230.

(Radio Shack Pocket Computer PC-1)