My shih tzu I used to cut their hair.
The reason is that though shih tzu in advertisement are traditionally kept with longer fur but because they are in cooler countries, while in Malaysia, it's a different climate altogether.

So keeping them short makes them much more comfortable. Unless you keep them in your house with the air-cond on 24 hours

All I used are just scissors. Trimming them.
Maybe becoz of the area I stay with garden, so keeping their fur short, makes movement easier and let's them enjoy playing outside.

But when trimming using scissors, make sure you know how to trim. Do not ever trim the front where his/her whiskers are. And also the legs are bothersome. Tail especially....do not cut the skin.