DLL and other 69,000 icons from Icons8 icon pack follow the visual guidelines of the operating systems: Windows, iOS, Android KitKat, and Material. PNG format is free up to 100x100 px. Icons are availble for download in PNG and vector (SVG, PDF, EPS, and more).
The shape of the DLL icon looks like a standard piece of white computer paper. The top right corner is folded down, similar to what one might do when bookmarking a novel they are reading. In the center of the rectangle are the uppercase letters DLL.