A .key file is a zip. Inside it, when the deck was saved with previews on, sits a PDF of every slide rendered by Keynote itself. Taking that out is an extraction rather than a conversion, so the fonts, the positions and the slide size are exactly what the author built.
Two things are not in it: presenter notes and separate pages for build stages. Both need an export from Keynote. If you need either, ask the sender rather than any tool.
The reason this works at all is that Apple needs it to work. Finder, Mail, Quick Look and iOS all have to show a deck without launching Keynote, so Keynote writes the render in as it saves. That render is a complete, ordinary PDF, and nothing about extracting it is clever.
What is inside a .key package
Index/*.iwa
The deck itself, in Apple's private binary format. Effectively unreadable outside Keynote.
QuickLook/Preview.pdf
Every slide, rendered by Keynote with the right fonts. This is the part worth having.
Data/
Images, video posters and audio placed on the slides, in their original form.
Metadata/
Which version of Keynote wrote the file, and the identifiers it syncs with.
The slides in the first entry are stored as Apple's own protocol buffers with an undocumented schema that changes between versions. Anything claiming to read that directly is guessing, and it will guess wrong about fonts it does not have. The second entry sidesteps the problem entirely by being a finished document.
A slide is not a page, and the shape shows it
The first surprise for most people is the size of the result. Keynote's default canvas is 1920 by 1080 points, a wide 16 by 9 rectangle, and the extracted PDF keeps it. Open that next to an A4 document and it looks enormous and badly proportioned, because it is a screen shape rather than a paper one.
Printing it is not a problem: any print dialogue will scale a slide to fit a sheet, and the result is a landscape slide with white bands above and below. What people usually want instead is a handout, several slides to a page, and that is a layout step rather than a conversion one. Run the extracted PDF through pages per sheet and pick two, four or six.
What the render leaves out
The preview is the deck as an audience sees it, which means two things an author might be looking for are simply absent.
Every slide, in order, at full quality.
Real text with embedded fonts, so it selects and searches.
Images, charts and shapes exactly as positioned.
The slide size the deck was built at.
Presenter notes, which live beside the slides and are never shown to an audience.
One page per build stage, so a slide revealed in five steps is one page.
Transitions and animations, which have no meaning on paper.
Any slide marked skipped, which Keynote leaves out of its own render too.
Both absences are Keynote behaving sensibly rather than a limitation of extracting the file. If you need the notes, the sender exports with Include presenter notes ticked. If you need the builds broken out, they export with one page per stage. Both are options in Keynote's PDF export and neither can be recovered from a package that does not contain them.
When the preview is not there
Some packages arrive without one. It happens when the deck was saved with previews switched off, when it came from an older version of Keynote that stored things differently, or when a mail client or file manager re-zipped the package in transit and dropped what it considered spare.
When that happens no tool can help, and it is worth being plain about why rather than offering something that will disappoint. The slides exist only in Apple's private format, and reading it requires Apple's software. The ten-second fix belongs to whoever sent it: open the deck, then File, Export To, PDF. On an iPhone or iPad, open it, tap the three dots, then Export.
If they can get to iCloud Keynote in a browser, which is free and needs no Mac either, they can export from there too.
Questions people actually ask
01Can I open a .key file on Windows?
Yes, when it was saved with previews on, which is the default. The package holds a PDF of the slides that Apple rendered, and the tool above extracts it. Nothing is converted, so the result is exact.
02Are the presenter notes in the PDF?
No. The render is the slides as an audience sees them. Notes need an export from Keynote with Include presenter notes ticked, so ask the sender for that if you need them.
03Why is my deck landscape and huge?
Because a slide is not a page. Keynote's default canvas is 1920 by 1080 points, so the PDF comes out at that shape rather than A4. It prints fine scaled to fit, and the page setup is the reason it looks oversized on screen.
04My deck has builds. Why is each slide only one page?
The preview renders each slide in its final state. Keynote's own export offers one page per build stage, which the preview does not, so a deck that relies on builds reads as a shorter document here.
05Can I get several slides on one page?
Not from the preview, which is one slide per page. Run the result through pages per sheet afterwards to get a handout with two, four or six slides on each sheet.
06What if there is no preview inside?
Then nothing outside Apple's software can read the deck, because the slides themselves are stored in Apple's private format. Ask the sender to export a PDF, which takes them about ten seconds.