That’s actually quite tough to pinpoint. Computer scientist Alan Kay first came up with a concept (and then a prototype) for what he called a ‘Dynabook’ in 1968. Depending on which version you look at, the Dynabook concept can be viewed as a prototype tablet PC (as well as a direct ancestor of the laptop).
In science fiction, tablet-like devices can be seen in ‘2001: A Space Odyssey’ as well as ‘Star Trek: The Next Generation’. While in comics, Jack Kirby’s ‘Motherboxes’ (as featured in the 1970’s ‘New Gods’ series) can be considered to be ‘super-tablets’ by any other name. So the idea for the tablet was firmly entrenched in fiction and popular culture long before the iPad was even a glint in Steve Jobs’ eye.
Jeff Hawkins developed the first modern-style tablet PC in 1989, this invention led to a prototype named Lombard (for some silly reason) that was released in 1992. However, before that, in 1987, Apple had designed hardware for a touchscreen and stylus operating system, which was a primordial version of the iOS that you would use today on the aforementioned iPad.
In 2002, Microsoft launched the ‘Tablet PC’, which was a grand idea on paper, but, for too many reasons to list here, the invention never took off. It would take ten long years (and the rapid rise of mobile phones) before Apple dusted off the idea and proudly produced their iPad, in 2010.
So, in a very real sense, no single person invented the tablet PC. It was a culmination of wild-eyed science fiction dreamers, wild-haired computer scientists and the market-driven will to profit as utilized by companies like Microsoft and Apple.
Personally, if I had to pick just one name out of the hat, it would be Alan Kay. Now, before all you tech-bods rush out to correct me, consider this: John Logie Baird invented the television, but his initial invention is barely recognizable compared with today’s net-ready, Blu-Ray playing, surround-sound enabled living room leviathans, so its just a question of who had the first idea.
I’ve seen sites that credit Jeff Hawkins, which is fair, but honestly, the idea (and an early version of the eventual tech) actually existed 30 years or so earlier, so I’m not going to personally subscribe to that one.