A4 paper is a widely used paper size, its specification is defined by the International Organization for Standardization ISO 216, the specific size of 210mm x 297mm. This size is both portable and suitable for most printers and copiers, so it is widely used in daily life and work. The name A4 comes from the fact that its size is the result of four folds of A0 paper, where the "A" represents the "ARCHES" series and the "4" represents the size of the paper, hence the name A4. The popularity of A4 paper is due to its status as an international standard, which was originally incorporated into the DIN standard by the Weimar Republic in 1922 and subsequently became the paper size standard adopted by most countries in the world. The size of A4 paper makes it ideal for printing documents, copying materials or writing letters, and it is an indispensable office supply for both individuals and businesses. The quality of the paper is also an important consideration when choosing A4 paper. High-quality A4 paper should have a slightly yellow color rather than too white to reduce the visual impact; The paper distribution is uniform, there is no obvious difference in thickness; The whiter the ash color after burning, the better the quality of the paper.