What is an ELM File?

How to Open, Edit, & Convert ELM Files

ELM file

A file with the ELM file extension is an Office Theme file. These are settings files used by Microsoft Office programs and Microsoft FrontPage.
An ELM file is an uncompressed file that holds all the different parts of the theme. They may also reference external files like JPGs or other images.
The fantasy MMORPG video game Eternal Lands uses the ELM file extension too, for Eternal Lands Map files. They are sometimes stored with GZ compression, and are therefore named *.elm.gz.
Note: Though the file extensions look awfully similar, ELM files are entirely different than EML (E-Mail Message) files.

How To Open an ELM File 

ELM files are used by Microsoft Office programs but can't be opened directly by them. In other words, even though you may have ELM files in your Microsoft Office installation directory, you can't manually open one in Word or Excel, for example.
Note: Microsoft Office 2013 keeps ELM files in the \Program Files\Common Files\microsoft shared\THEMES15\ folder. Office 2010 uses the \THEMES14\ folder and ELM files under Office 2007 are kept in the same path but under the \THEMES12\ folder.
The now discontinued Microsoft FrontPage web design program uses ELM files too.
Since Office Theme files are normally entirely text-based, any text editor can open them too, like Notepad++ or Windows Notepad.
ELM files opened this way don't let you use the file like you'd expect, but instead just shows some detail about the theme in text form.
The free Eternal Lands game uses ELM files that are Eternal Lands Map files.
If you find that an application on your PC does try to open the ELM file but it's the wrong application or if you would rather have another installed program open ELM files, see my How To Change the Default Program for a Specific File Extension guide for making that change in Windows.

How To Convert an ELM File 

ELM files used by the aforementioned Microsoft products probably cannot be converted to any other format and still do what they do. They're used by the appropriate programs automatically, and those only, so conversion to a different format is unnecessary.
If for whatever reason you do want to convert an ELM file to something like HTM, TXT, or another text-based format, you can do so with a text editor. But again, this would produce a file that would no longer function properly with Microsoft products and would only be useful to make it easier for you to read the text contents of the file.
Similarly, the Eternal Lands game is probably the only other software that uses ELM files. Since they're of an entirely different format from Office Theme files, they most likely need to remain in their original format (with the .ELM extension).
Important: You cannot usually change a file extension (like the .ELM file extension) to one that your computer recognizes (like .JPG) and expect the newly renamed file to be usable. An actual file format conversion using one of the methods described above must take place in most cases.