Microsoft Word (Office Open XML)

DOCX is the file format of Microsoft Word since Word 2007 and the format most documents are exchanged in today: CVs, theses, contracts, templates from employers and schools. LibreOffice, Google Docs and Apple Pages all open and save it, so a .docx usually works even if neither you nor the sender has Word.

Origin and standard

Microsoft designed Office Open XML as the successor of the binary DOC format and had it standardised by Ecma International as ECMA-376 in December 2006. In 2008 it became the international standard ISO/IEC 29500. The standard has two flavours: Transitional, which keeps legacy features for compatibility and is what Word writes by default, and Strict, which drops them. Word 2013 and later can save Strict files (Save as > “Strict Open XML Document”), but some older or simpler programs do not read them, so Transitional is the safer choice for sharing.

How a DOCX works inside

A .docx is a ZIP archive. Rename a copy to .zip and you can look inside:

• word/document.xml holds the text, organised into paragraphs (w:p), runs of equally formatted text (w:r) and the text itself (w:t).
• word/styles.xml, numbering.xml, headers, footers, footnotes and comments sit in separate XML files.
• word/media/ contains images in their original format. To get every picture out of a document at full resolution, unzip it and copy this folder.
• docProps/core.xml stores metadata: author, last modified by, creation and revision dates. Check it before sending a document outside your company.

Thanks to ZIP compression a DOCX is usually smaller than the same document as DOC. A plain .docx cannot contain macros; Word refuses to save VBA into it and uses .docm instead. Fonts are normally only referenced by name. Word can embed them (File > Options > Save > Embed fonts in the file), but few people switch that on, which is why documents look different on other computers.

How to open and edit DOCX

• Windows and macOS: Microsoft Word; free alternatives are LibreOffice Writer and, on a Mac, Pages.
• Linux: LibreOffice Writer.
• Browser: Word for the web (free with a Microsoft account) or Google Docs, which can also edit a .docx without converting it.
• Phone: the Word app for Android and iOS, Google Docs; on an iPhone the Files app shows a preview.

Common problems

DOCX won’t open: “Word found unreadable content” or “the file is corrupt”

Because a DOCX is a ZIP archive, an interrupted download or a truncated mail attachment damages the whole package. First check the file size; 0 KB means the content is simply missing. Otherwise try Open and Repair in Word (File > Open, arrow next to Open). LibreOffice is often more tolerant of broken packages, and as a last resort you can unzip the file and read the text from word/document.xml.

The document looks different on another computer

Almost always a font issue. Microsoft 365 replaced Calibri with Aptos as its default font in a rollout that began in 2023. A recipient with older Word, LibreOffice or a phone without Aptos gets a substitute with different widths, and lines and pages break elsewhere. If the layout must stay fixed, send a PDF.

A DOCX opens as a ZIP folder or in the wrong program

The file association is wrong, or a browser or mail system changed the extension. Right-click the file, choose Open with, select your word processor and tick “Always use this app”.

Word 2003 cannot open DOCX

Word 2000 to 2003 needed the Microsoft Office Compatibility Pack, which is no longer offered. Convert the file to DOC for such machines.

Converting DOCX

The most frequent conversion is DOCX to PDF for sending and printing; the reverse direction, PDF to DOCX, makes a PDF editable again. Our converter runs on our own servers and can also turn a DOCX into DOC, ODT, RTF, TXT or HTML, into EPUB for e-readers, and into Markdown. Text, styles, tables, images, footnotes, comments and tracked changes survive conversion into DOC and ODT. When rendering to PDF, the server uses metric-compatible replacements for Calibri, Cambria, Arial and Times New Roman, so page breaks stay put; documents set in Aptos or other commercial fonts may reflow slightly. Remove password protection before uploading. Older files can be upgraded with DOC to DOCX.

DOCX in brief

A detailed technical description is published by the Library of Congress.