DOCX vs DOC: What's Actually Different (and When It Matters)
Both open in Word, but DOC and DOCX are built on completely different technology. Here's what that actually changes for you.
Two different formats, one program
DOC was Microsoft Word's format from Word 97 through Word 2003: a single binary file that only Word itself, or software specifically built to parse that binary structure, could reliably read.
DOCX replaced it starting with Word 2007. It isn't a new version of the same format -- it's a completely different approach: a DOCX file is actually a ZIP archive containing a set of XML files (the "Office Open XML", or OOXML, standard). Rename any .docx file to .zip and a normal file archiver will open it and show you the XML inside.
Why Microsoft made the switch
XML-based formats are openly documented, so other software -- Google Docs, LibreOffice, Apple Pages, and the conversion tools on this site included -- can read and write them accurately without reverse-engineering a proprietary binary layout. ZIP/XML-based files also compress well and are less prone to total corruption: damage to one part of the XML often leaves the rest of the document recoverable, which was much harder with the old binary format.
When the difference actually matters to you
Compatibility with older software. Word 2003 and earlier can't open a .docx file without a separate compatibility pack -- the most common real-world reason someone still needs a plain .doc.
File size. A DOCX file is typically smaller than the equivalent DOC file, because the underlying XML compresses well inside the ZIP container.
Features. Several newer Word features were introduced alongside the DOCX format and have no clean equivalent in the older DOC structure.
For most everyday use -- writing, editing, sharing with someone on a reasonably current version of Word or Google Docs -- the difference is invisible. It mainly surfaces when dealing with an older system, or converting the file to something else, like PDF.
Converting either one to PDF
Our Word to PDF tool accepts both .doc and .docx and converts either to a fixed-layout PDF that looks the same regardless of which Word version -- or whether Word at all -- the recipient has installed.