iZotope RX is audio editing restoration software.
RXDOC files are the RX Document file format which "includes your original file, all the edits you’ve made to it, and your most recent selection and view state. RX Documents can only be opened in the RX Audio Editor."
RXDOC files begin with the hex values
22C8C934 for all versions. They also contain XML at the end with metadata for the project.