1. An image processing device comprising: a processor to perform: acquiring target image data representing a target image including a letter; and generating binary image data representing the letter in the target image by using the target image data, the generating of the binary image data comprising: identifying a background color value representing color of background of the target image; identifying a letter color value representing color of the letter in the target image; determining a characteristic value concerning a sharpness of the letter in the target image; determining a binarizing threshold value by using the background color value, the letter color value, and the characteristic value; and performing a binarizing process on the target image data by using the binarizing threshold value to generate the binary image data.