Information processing device, method, and a non-transitory computer readable storage medium for scanning an image using a protective sheet to estimate a ratio of document lengths, acquired from scanned images, to correct a length of a scanned image or length of a cropped image, based on the ratio

An information processing device executes: acquiring a scanned image of a document and a protective sheet that protects the document, the scanned image being obtained by imaging the document and the protective sheet while being conveyed at the same time; acquiring a first length that is a conveying direction length of a predetermined portion of the protective sheet; acquiring a second length that is a conveying direction length of the predetermined portion in the scanned image; and estimating a ratio between a document length, which is a conveying direction length of the document, and a document image length, which is a conveying direction length of the document in the scanned image, based on the first length and the second length.