Files created by Adobe Illustrator come as AI format and they are single-page drawings based on vectors. The AI files consist of paths connected by points, rather than bitmap image data.
There are several applications that support AI format which can enlarge these files with no quality loss.
Adobe Illustrator is the recommended application for opening this file type, but there are alternatives to this software.
Nonetheless, some graphic viewers might rasterize the image, thus affecting the image quality.
Users need to be aware that only specialized applications will be able to handle the image layers from AI files.