PDX file format is used by Adobe Acrobat software application. The user can make an index of a specific PDF file and the index when made is stored in separate file as PDX file.

This file allows the application to search within the file faster. This index file is usually embedded in the file for fast searching process. If the PDX file is embedded in the main document the application searches in index rather than looking in the main document and hence it takes less time for the application to get the results when the user searches for any specific element in the PDF document.