What is an NCX file? Table of contents file created according to the XML-based Open Packaging Format (OPF) standard, which is part of the EPUB eBook. The NCX file type is primarily associated with SecurDataStor by encryptX Corporation. How to open NCX files. NCX stands for EPUB Navigation Control XML File. Follow These Easy Steps to Open NCX Files · Step 1: Double-Click the File · Step 2: Choose the Right Program · Step 3: Figure Out the File Type · Step 4: Check with.