There are several options that might be appropriate for this student or for others in a similar situation. Some scanners come with software enabling the user to scan directly into a PDF document; however, it is more likely that you will have to purchase either Adobe Acrobat or third-party software that will allow you to convert scanned documents into PDF.
Converting the scanned image would enable you to maintain the original layout of the document and still work with Natural Reader since it is capable of reading PDFs as well as MS Word documents. Having the capability to convert documents to PDF could also be beneficial for other students, as the newer versions of Adobe Reader have improved read out loud capabilities. This could be helpful for students who don't have access to a screen reader at home. You could convert any text to a PDF and students could hear it read aloud using the free Reader program.
If purchasing additional software is not a feasible option, you may also try searching for a digital version of the text online. Learning Ally has audio versions of many textbooks, and websites such as BookShare and Project Gutenberg have electronic books freely available for download (BookShare provides books free for users with documented print disabilities).