CDK File

What are CDK files and how to open them

Are you having problems opening a CDK file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your CDK files.

What is a CDK file?

A .CDK file is a Calamus Document file.

Calamus Document files store full page layouts created using the Calamus desktop publishing program, originally developed by DMC GmbH for Atari ST and later versions including Calamus SL. These files hold all elements of a multi-page publication - text frames (flowed or static), graphics frames (raster or vector), layout guides, font and style information - so that complete documents can be edited, printed, exported or converted.

Such files are produced when a user designs pages with Calamus. After arranging frames, importing text or images, selecting fonts and applying styles, the document is saved in this native format. These files are then re-opened in Calamus to edit or revise, and are often exported to common formats (ASCII, PETSCII, GEM-Image etc.), converted to PostScript or used for printing services.

Native Calamus Documents begin with a distinctive binary signature. The first bytes are ASCII "DMCCALAMUS" at offset 0, and at offset 13 the sequence bytes 0x43 44 4B 01 (ASCII "CDK" + version byte) appear. This identifier helps software recognize authentic Calamus layouts.

How to open CDK files

Important: Different programs may use files with the CDK file extension for different purposes, so unless you are sure which format your CDK file is, you may need to try a few different programs.

While we have not verified the apps ourselves yet, our users have suggested five different CDK openers which you will find listed below.

Last updated: August 31, 2026

Various apps that use files with this extension

These apps are known to open certain types of CDK files. Remember, different programs may use CDK files for different purposes, so you may need to try out a few of them to be able to open your specific file.

Help us help others

File.org helps thousands of users every day, and we would love to hear from you if you have additional information about CDK file formats, example files, or compatible programs. Please use the links below or email us at submit@file.org to discuss further.