PDX files are index files of the Acrobat document software. Pdx files include the index of PDF documents and folders that contain documents. With the help of the PDX file, Acrobat can quickly search many pdf files.
PDX files can be created under 'Advanced-> Document Processing-> Full Text Index with Catalog' in Acrobat. In addition to a PDX file, Acrobat also creates a folder of the same name with idx files that contain the actual index entries. It should be noted that when changing the folder structure, the index may need to be recreated.
Index files can also be used by Acrobat Reader, but the associated PDF documents and idx files must exist in the original folder structure.