For most people, the PLR-16 is a fun range blaster, and it's great at that, but Kel-Tec designed and marketed it as a Pistol Long Range, and it's great at that too. It's fun to shoot from the bench at 100 yards. For that, you'll want a real rifle scope. The 3-12X you described sounds pretty good. The difficulty arises because the pistol doesn't have a butt stock so there is no cheek rest. This makes it difficult to get a consistent image through a high power rifle scope. You can have greater parallax error than you'd have when shooting a rifle. You can probably come up with a few tricks to help with the eye alignment to minimize this source of inaccuracy. Try to have some method that aligns your eye a fixed distance from the scope's objective lens. I've found that it helps to have your eye far enough away that you are just starting to get a dark ring around the outside of the field of view. If you move your eye to the position that makes the dark ring uniform around the outside, your pupil should be well aligned with the center of the scope, but it's still a bit difficult to hold it there while shooting while also aligning the reticle with the target, controlling your breathing, concentrating on a smooth trigger pull, etc.
A lower power scope can also be useful for shooting the PLR-16 as a pistol. In that case, you'll want an extended eye relief (EER) scope, sometimes called a scout scope or a handgun scope. A shoulder sling will help stabilize the PLR-16 pistol for best accuracy when shooting it scoped. The sling is not as good as a butt stock, but it's better than trying to hold this large pistol steady at arm's length without a shoulder sling.