PSD (Photoshop Document), AI (Adobe Illustrator), EPS (Encapsulated PostScript) are all the graphics file formats created by Adobe Systems company, used for Adobe Photoshop and Adobe Illustrator. They store layers, channels, reference lines, annotations, color modes and other information added by Photoshop/Illustrator. Although, we can also use ACDSee such as this universal image viewer, that way is not fast and convenient enough.