In recent years, digital notebooks have been replacing traditional paper-based notebooks with the development of handwriting input devices. Currently, we can access digital notebooks in various devices, including mobile devices. When we use such mobile devices, however, their limited screen size results in difficulty in understanding the summary of hand-written documents, without the use of a zoom feature. In this paper, we therefore propose the "Legible Thumbnail" that helps us to understand the summary without zooming. Our method detects the important words based on emphasis, such as an underline, and the method outputs the emphasized words to the thumbnail. Experiments show our thumbnail reduces search time by 21%.