What is IDML, and how is it different from a PDF?
Last updated June 2026 · by Kelly Fraser, maker of idmly
IDML is InDesign Markup Language: an open, XML-based, fully editable format for Adobe InDesign layouts. An .idml file is a zipped package of XML that describes a document's text frames, named styles, colours and objects, everything InDesign needs to rebuild the layout as a live, editable document.
That's the opposite of a PDF, which is a flattened, final-form file built to look identical everywhere, not to be edited.
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.
IDML vs PDF
| IDML | ||
|---|---|---|
| Purpose | Editable layout interchange | Fixed final-form display |
| Text | Live, in named styles | Flattened, often outlined |
| Editable in InDesign | Fully | No, placed as one graphic |
| Under the hood | Zipped XML | Flattened page description |
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.
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
What does IDML stand for?
Can InDesign open an IDML file?
.idml directly (File, Open) and creates a fully editable, untitled document. Affinity Publisher can open IDML too.What's the difference between IDML and INDD?
Is an IDML file editable?
How do I create an IDML file from an HTML or AI design?
.idml directly, with real text frames, named styles and editable charts. No PDF step, no rebuilding by hand.Is an IDML file the same as an Adobe Illustrator (.ai) file?
.idml) is InDesign's markup format; Illustrator's format is .ai. They are unrelated. When idmly converts an "AI design" it means an AI-generated HTML layout, not Illustrator artwork.Want a real .idml from your design?
Drop your HTML in and the first two pages come back as a native, editable InDesign file, free.