Both are “Word documents” in casual conversation, but .doc and .docx are genuinely different file formats under the hood, not just an updated file extension on the same thing — and that difference is where a surprising number of “why won’t this open” problems come from.
| .doc | .docx | |
|---|---|---|
| Word versions | Up to Word 2003 | Word 2007 onward |
| Underlying structure | Proprietary binary | ZIP archive of XML files |
| File size | Larger for equivalent content | More space-efficient |
| Damage recovery | Hard to partially recover | Undamaged XML parts often stay readable |
| Third-party support | Reverse-engineered | Open spec, easier to read reliably |
.doc stores content in a proprietary, compressed binary structure that isn’t human-readable and requires Word’s own logic (or software specifically built to parse it) to unpack. .docx, introduced with Word 2007, is built on Office Open XML — under the hood, a .docx file is actually a ZIP archive containing a set of XML files describing the document’s text, formatting, and structure, plus any embedded images. You can, technically, rename a .docx to .zip and open it in a file archiver to see this directly. That XML-based structure is why .docx generally wins on every row of the table above: XML compresses well and the format was designed with this in mind, a damaged ZIP-based file often has undamaged XML parts that stay readable even when part of the archive is broken, and third-party software reads it more reliably since the specification is open rather than reverse-engineered.
Why this still causes problems
The practical friction shows up in a few specific ways. Modern Word occasionally struggles with very old .doc files, particularly ones written by long-discontinued word processors that only loosely followed the .doc spec, or files with unusual embedded objects from software that no longer exists. Some older or lightweight software only handles one format — a basic viewer or an older third-party tool built years ago might support .doc but not the newer .docx, or vice versa depending on when it was written. And macros change the extension entirely: a .docx containing macros isn’t actually valid, it has to be saved as .docm, a related but distinct format Word treats differently for security reasons, since macros are a common malware vector and Word flags .docm files for extra scrutiny on open.
Converting between them
Word itself handles conversion natively — File → Save As, then pick the format you need, which is the most reliable route since it’s the software that defined both formats. If all you need is the format changed with nothing else touched, DOC to DOCX does exactly that: upload the old .doc file and download it as .docx, no Word required. This site’s other tools also work with both without you needing to think about which one you have — Compress Word and Merge Word Documents both accept .doc and .docx interchangeably, and normalize a .doc to the modern .docx format as part of processing it, so an old .doc that’s giving you compatibility grief elsewhere comes back in the more broadly compatible format.
If the document’s destination is being read rather than edited — a CV, an invoice, a signed agreement — converting to PDF with Word to PDF removes the .doc-vs-.docx question for the recipient altogether. A PDF opens identically regardless of which Word format produced it, on any device, with no compatibility surprises for whoever’s on the other end.
Related reading: Why Is My Word Document So Large? covers six causes of Word bloat, independent of which format you’re using. How to Merge Word Documents Online covers combining files that don’t even share the same format. How to Submit Documents for a Job Application covers where a portal saying “.docx only” bites hardest. How to Convert a PDF Resume to an Editable Word Document covers the other direction, when the only copy you have is a PDF. And How to Convert an Old .doc File to .docx is the plain walkthrough for just the format swap, step by step.
Bring an old file up to the modern format with the free DOC to DOCX converter — or use Compress Word and Merge Word, which handle both formats the same way. No sign-up, no watermark, nothing stored.