How to print a A4 format PDF on a letter size paper which centre of the doc is printed in the middle

It depends on what operating system you are using. You don't say, so we have to guess. Someplace on your computer there is a menu icon. Click that and hunt around until you find "Printers". While you are hunting, notice what other menus and options you have in case you want to change them later. You should do the same in every program that you use. Any program that allows to print also allows direct access to the printer page setup options. This is called "learning to operate the computer."