Saving to Older Versions of InDesign
See Chapter 12: “Collaboration” from Mastering InDesign CS3 for Print Design and Production by Pariah S. Burke (Sybex, 2007).

Not all of us upgrade as quickly or regularly as others. For some, InDesign CS2 is ideal, with all the features needed for their particular work; they don’t need or want CS3 and will not upgrade to it for a while if ever. (I doubt you are included in this group; after all, you obviously bought a book titled Mastering InDesign CS3 for Print Design and Production.) Others may lust after a new version but simply can’t justify its price tag. Whatever the reason, it becomes necessary on occasion to move documents between the latest and earlier versions of InDesign.

The fact that InDesign CS (CS1, version 3.0) did not save backward for compatibility with 2.0 (the first commercially successful edition of InDesign) brought forth a public outcry so vociferous that it still echoes throughout the halls of Adobe and many press and pre-press shops around the world. Allow me to dispel another common misconception: Despite what you may have heard, InDesign does save backward. It has done so since CS2, which saved documents compatible with CS1.

To save a document such that it can be opened in CS2, choose File > Export. In the Export dialog, change the Save as Type drop-down to InDesign Interchange format. When you save the document, it will be with the .inx file extension. InDesign Interchange documents are XML based, and may be opened in InDesign CS2 or CS3 with the File > Open command. Both versions can also export them, although CS2 users sending documents to CS3 users can simply send documents in the standard INDD format. CS3 will open documents created by any prior version of the program. It will only save INDD files as CS3 version, however.

Maintaining a round-trip editing workflow between InDesign CS2 and CS3 is tedious, but doable. The CS2 user can send his work as either INDD or INX formats, but the CS3 user must send her work back as INX files, which the CS2 user can open and edit.

Note: An update to InDesign CS2 was released shortly prior to the release of CS3. Contained in the update was a new version of the INX filter that enables CS2 to open CS3-authored INX documents. If you or a CS2-based associate experiences any problems opening such documents, ensure that InDesign CS2 has been updated to the latest version. You can do that by opening InDesign and choosing Updates from the Help menu. The Adobe Updater utility will then check for recent updates to InDesign and all installed Adobe applications and present you with a list of available downloads. Install any available InDesign updates. While you’re at it, it’s a good idea to install available updates and patches for all your Adobe applications.