DOCX is the default file format for Microsoft Word documents since 2007. While most people treat it as a black box — you create a document in Word, save it, and open it later — understanding what a DOCX file actually contains helps explain why some conversion tools produce better results than others, especially for mathematical content.
DOCX Is a ZIP File
Here is something most people do not realize: a DOCX file is actually a ZIP archive containing a collection of XML files and media assets. If you rename any .docx file to .zip and extract it, you will find a structured folder containing:
word/document.xml— The main document content (text, paragraphs, formatting)word/styles.xml— All style definitions (heading styles, normal text, etc.)word/media/— Any embedded images[Content_Types].xml— Metadata about the file types contained in the archiveword/_rels/— Relationship files that link document parts together
This format is officially called Office Open XML (OOXML) and is standardized as ISO/IEC 29500. The "Open" in the name refers to the fact that the format specification is publicly available, allowing any software to read and write DOCX files.
How DOCX Stores Math Equations
This is where it gets interesting for anyone working with mathematical content. DOCX stores equations using Office Math Markup Language (OMML), which is an XML vocabulary specifically designed for representing mathematical notation.
When you type an equation in Word's equation editor, Word generates OMML XML that describes the equation's structure. For example, a simple fraction like "1 over 2" is stored roughly like this:
<m:f>
<m:num><m:r><m:t>1</m:t></m:r></m:num>
<m:den><m:r><m:t>2</m:t></m:r></m:den>
</m:f>
The m:f element represents a fraction structure. m:num contains the numerator and m:den contains the denominator. This structural representation is what makes the equation editable — Word's equation editor can read this XML and present it visually, allowing you to click into it and modify the numerator or denominator.
Why This Matters for Conversion Quality
When a conversion tool produces a DOCX file, there are fundamentally two ways it can handle equations:
Method 1: Equations as Images (Bad)
The lazy approach is to render each equation as a picture (PNG or EMF) and embed it in the DOCX file as an image. The equation will look correct when you open the document, but it is not editable. You cannot click into it, change a variable, or modify a coefficient. If you need to make changes, you have to delete the image and retype the equation from scratch.
This is what most generic PDF-to-Word converters do. The equation "survives" visually, but it is a dead end for editing.
Method 2: Equations as OMML (Good)
The proper approach is to generate OMML XML that describes the mathematical structure of each equation. When you open the resulting DOCX in Word, you can click any equation to enter the equation editor and modify it freely. The equation is a live, editable object — exactly as if you had typed it using Word's equation editor yourself.
This is what MathToWord produces. Our AI recognizes the mathematical content, converts it to an internal representation, and then generates the corresponding OMML XML embedded in a properly structured DOCX file. The result is a document where every equation is fully editable.
DOCX vs DOC: The Important Difference
The older .doc format (used by Word 97-2003) is a binary format — it cannot be opened and inspected as XML. It also uses a completely different equation system (Microsoft Equation 3.0, which relied on an OLE object). The modern .docx format with OMML equations is significantly more capable, more interoperable, and more future-proof.
If you are still working with .doc files, converting them to .docx (File → Save As → Word Document) is strongly recommended. The OMML equation system in DOCX is both more powerful and better supported by modern tools.
Compatibility Across Applications
DOCX files with OMML equations are best viewed and edited in:
- Microsoft Word (desktop): Full support for creating, editing, and rendering OMML equations.
- Microsoft Word (web/online): Can display OMML equations but editing support is limited.
- LibreOffice Writer: Can display most OMML equations, but complex ones may render differently. Editing support is partial.
- Google Docs: Can display OMML equations as read-only images. Cannot edit them.
- Apple Pages: Limited OMML support. Some equations may not display correctly.
For the best experience with mathematical DOCX files, Microsoft Word on desktop (Windows or Mac) remains the gold standard. The web version and third-party applications have improving but still incomplete support.
File Size Tip
Because DOCX is a ZIP archive, documents with OMML equations (XML text) are typically much smaller than documents with equation images. A 50-page document with hundreds of OMML equations might be 200 KB, while the same document with image-based equations could be 5-10 MB. This also makes OMML-based documents faster to open, save, and share.
Conclusion
Understanding the DOCX format helps explain why conversion quality varies so dramatically between tools. The file format itself supports rich, editable mathematical content through OMML — but only tools that generate proper OMML produce documents where equations can actually be edited. When choosing a conversion tool for math-heavy documents, always verify that the output contains real equation objects, not embedded images. You can test this easily: try clicking on an equation in the output document. If it opens in Word's equation editor, it is OMML. If it selects as an image, it is not.