What is IDML, and how is it different from a PDF?

If you've ever received an .idml and weren't sure what it was, here's the short, useful version, and why it's the format idmly produces instead of a PDF.

What's inside an IDML file

An IDML is really a ZIP archive of XML documents. Rename one to .zip and you'll find the spreads, stories (the text), styles, and resources laid out as structured data. Because it's XML, it can be read, written and generated by software, not just by InDesign itself. That's exactly how idmly builds one: it writes valid IDML directly from your rendered HTML layout.

How to open an IDML file

Three layout apps open an .idml as a live, editable document. If you have none of them, you can still get at the text.

• →Adobe InDesign (CS4 or later). Go to File > Open and pick the .idml. InDesign builds a new untitled document from it. Save that as .indd and carry on.
• →Affinity Publisher. File > Open works the same way, with no Adobe subscription needed.
• →QuarkXPress (2017 or later). File > New > Project from IDML converts it into a QuarkXPress layout.
• →No layout app at all. Rename the file from .idml to .zip and unzip it. The text sits in the Stories folder as XML. You can read it, but you can't edit the layout this way.

Two things to expect when it opens. Text highlighted in pink means a font the file names isn't installed, so install or activate it and the highlight goes. A missing-links warning means the images aren't where the file expects them: keep the Links folder beside the .idml, as it arrived.

IDML vs PDF

IDML vs INDD

.indd is InDesign's native, binary document format, and it's tied to specific InDesign versions, so a newer file won't open in an older InDesign. .idml is the open, portable interchange format: more version-flexible, and generatable by software. When you open an IDML, InDesign creates a fresh untitled document you then save as INDD. Affinity Publisher can open IDML too. The full side-by-side, including which one to send and the mistakes to avoid, is in IDML vs INDD.

Why it matters for your work

If you want a design you can keep working in, reflow text, swap a font, recolour a chart, you want IDML, not a placed PDF. That's the entire reason idmly outputs a native .idml: so what you get back is a real document, not a picture of one.

Where an IDML file comes from

InDesign and Affinity Publisher can both export IDML, but you can also generate one from a design that never lived in either app. That's what idmly does: it takes a self-contained HTML layout, the kind AI design tools hand you, and writes a valid .idml straight from the rendered page. So a design built in Claude, ChatGPT, v0, Lovable, Bolt, Gemini, Figma Make, or plain hand-written HTML becomes a real, editable InDesign file, with no PDF in between and no manual rebuild.

One naming note, since it trips people up: an .idml is not an Adobe Illustrator file. "AI" in "AI design" means an AI-generated layout, not Illustrator's .ai format. IDML is InDesign's interchange format; idmly converts the HTML those AI tools produce into it.

Frequently asked questions