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.
What actually changed
.doc is the older, binary format used by Word versions up to 2003. It 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.
The XML-based structure is why .docx is generally the more space-efficient of the two for equivalent content (XML compresses well, and the format was designed with this in mind), more resistant to corruption (a damaged .doc’s proprietary binary structure is harder to partially recover than a ZIP-based format, where undamaged XML parts often stay readable even if the archive is partly broken), and easier for third-party software to read reliably, since the format specification is open rather than reverse-engineered.
Why this still causes problems
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.
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 (macros are a common malware vector, so Word flags .docm files for extra scrutiny on open).
Converting between them
Word itself handles this natively: File → Save As, then pick the format you need — this is the most reliable route since it’s the software that defined both formats. This site’s 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.
When to sidestep the format question entirely
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? — six causes of Word bloat, independent of which format you’re using
- How to Merge Word Documents Online — combining files that don’t even share the same format
Ready? Try the free Compress Word tool or Merge Word tool — both handle .doc and .docx the same way, no sign-up, no watermark, and your file is never stored.