DJVU usage

DJVU is a document file format.For scanned document, it often has higher compression rate smaller file size compare to PDF file with comparable quality. There are open source djvu library. But since its GPL license, by its requirement, including it in the MXTNote app will require us to make the app GPL license, which is not in our current plan. So currently we can only support converting DJVU to PDF by our server side API.