If a Word-to-PDF tool tells you it only accepts .docx and your file ends in .doc, you’ve run into a real but easily fixed difference. The two look almost identical and both open in Word, yet under the hood they’re built completely differently. This short guide explains why the newer format is required, how to check which one you have, and the fastest ways to upgrade an old file before converting.
What’s the difference?
.doc is the original Word format, used by default up to Word 2003. It’s a compact but complicated binary format — essentially a private Microsoft encoding that’s hard for other software to read reliably.
.docx arrived with Word 2007 and has been the default ever since. It’s an open, ZIP-based format: unzip a .docx and you’ll find a tidy set of XML files describing the document’s paragraphs, styles, tables, and images. Because the structure is open and documented, other programs — including a browser-based converter — can read it predictably. That openness is exactly why in-browser tools support .docx and not the older .doc: the modern format can be parsed accurately on your own device, without a server.
| .doc | .docx | |
|---|---|---|
| Introduced | Older Word (up to 2003) | Word 2007 and later |
| Format | Closed binary | Open, ZIP + XML |
| Read by other apps | Unreliable | Reliable |
| Works in browser converters | Generally no | Yes |
How to tell which one you have
The quickest check is the file’s extension — the bit after the dot in its name. On a
computer you may need to switch on file extensions first: on Windows, open File Explorer, go to the
View menu, and tick “File name extensions”; on a Mac, Finder → Settings
→ Advanced → “Show all filename extensions.” Then look: report.doc
is the old format, report.docx is the new one. If Word’s title bar shows
“Compatibility Mode” next to the filename, you’re almost certainly working with a
.doc.
How to convert .doc to .docx
In Microsoft Word (easiest)
- Open the .doc file in Word.
- Choose File → Save As.
- In the file-type dropdown, pick Word Document (.docx).
- Save. You’ll now have a .docx copy ready to convert to PDF.
Word may also offer a Convert button under File → Info that upgrades the document in place — that works too.
In Google Docs (free, no Word needed)
- Upload the .doc to Google Drive and open it with Google Docs.
- Choose File → Download → Microsoft Word (.docx).
This is a handy route if you don’t own Word. One caveat: Google Docs re-lays-out the document as it imports, so very complex formatting can shift slightly — open the result and check it before relying on it.
On a phone
The Microsoft Word mobile app can open a .doc and save a .docx copy via its share/save options, and the Google Docs app can do the same through Share & export. Once you have the .docx, you can convert it to PDF right there in your mobile browser — see converting on your phone.
What about even older or unusual files?
Files like .rtf (Rich Text Format) or .odt (OpenDocument, from LibreOffice) follow the same idea: open them in Word, Google Docs, or LibreOffice and re-save as .docx. Once a document is in the modern Word format, it’s ready for a clean, in-browser conversion to PDF.
The rule is simple: get your document into .docx first, and the rest just works.
With a .docx in hand, drop it into the converter and download your PDF — entirely in your browser, with nothing uploaded.