Select the input text, right-click the paragraph, set the alignment to center, and realize the center by changing the top, bottom, left and right line spacing. You can check whether it is centered in the preview, and then click OK.
A4 paper is defined by ISO 2 16, and its specification is 21* 29.7 cm (210 mm× 297 mm). Most countries in the world use this international standard for paper size.