I may have not used the right description in the posting above; by 'right angle joint' I don't mean the corners of the structure, but rather where a shelf butts into a vertical and is held in place by some pocket screws. Also, I may make the shelves out of 1/2 ply instead of 3/4 ply, if that makes a difference.

Will pocket screws hold?
Probably, but they are already installed in a compromised position upward from the bottom of the shelf.

Why not cut grooves in the sides of the case and house the shelves?
This way you get the full depth of the shelf to transfer the load.

The grooves can be cut with a router easily.

I don't see any advantage of using pocket hole joinery for the application you describe. This would be about as strong as nailing through the vertical sides into the shelves- but more work and leaving visible pocket holes in the bottoms of the shelves.

The strongest construction would be dado joints, glued and nailed. I use pocket hole joinery to assemble the face frame prior to attaching it to the plywood cabinet box. The pocket holes are concealled in the back of the face frame. For high end cabinets, I use the biscuit joiner to attach the face frame to the cabinet so there are no nail holes visible in the front- And no chance of shiners sticking out inside the box.

Thanks for the advice. If I do get a Kreg outfit, I'll just use it for face frame assembly and secure the shelves some other more secure method.

Or I could 'cheat' and just get some Knape & Vogt bookshelf standards...