An IDML file is a document created by InDesign, a program used to create professional page layouts. It contains several XML files, which represent a complete InDesign document. IDML files are stored as a compressed package using Zip compression.

IDML packages contain a hierarchy of XML files and directories. The root directory of the archive contains a designmap.xml file, which specifies high-level document properties, as well as the document layout. The archive also contains the following folders, which are used to store parts of the document: Resources: Contains document elements, such as colors, fonts, and paragraph styles; includes Graphics.xml, Fonts.xml, Styles.xml, and Preferences.xml. Spreads: Stores document spreads. Stories: Contains all document stories. XML: Stores document elements and settings; includes BackingStory.xml, Tags.xml, and Mapping.xml. MasterSpreads: Contains MaterSpreads.xml, which specifies all master spreads displayed on document pages, such as rectangles, ellipses, groups, polygons, buttons, and text frames. META-INF: Stores the standard container.xml file for UCF packages. IDML files were designed to replace .INX files, which are more difficult to create and edit. IDML files provide an interchangeable format for Adobe InDesign documents, as well as a way for third-party tools to modify and assemble InDesign documents. NOTE: The InDesign Markup Language was introduced with InDesign CS4. InDesign still supports backward compatibility with INX files.