File extensions are a secret language. Those three or four letters after the dot tell you what a file is, what can open it, and what it's good at. Here's a friendly tour of the ones you'll actually meet.
Formats fall into two camps: editable (built to keep working on) and fixed (built to share as-is). Match the format to which one you need.
The everyday document formats
| Extension | What it is | Reach for it when… |
|---|---|---|
| Portable Document Format — a fixed, print-ready snapshot | You want it to look identical for everyone | |
| .docx | Modern Microsoft Word document | You're writing or editing text |
| .doc | The old (pre-2007) Word format | You're dealing with a legacy file — convert it |
| .rtf | Rich Text Format — simple formatted text | You need basic formatting any editor can open |
| .txt | Plain text, no formatting at all | You want raw text, code, or maximum compatibility |
| .odt | OpenDocument Text (LibreOffice/OpenOffice) | You use open-source office software |
The "office suite" siblings
Word isn't the only document. Its siblings share the same modern XML-based design (that's the x in the extension):
- .pptx — PowerPoint presentations (slides).
- .xlsx — Excel spreadsheets (numbers, formulas, tables).
- .docx — Word documents (prose and reports).
The trailing x was a big deal. In 2007 Microsoft switched from secret binary formats (`.doc`, `.ppt`, `.xls`) to open, zipped XML (`.docx`, `.pptx`, `.xlsx`). If you ever rename a `.docx` to `.zip` and unzip it, you'll find the document is really a little folder of XML files and images.
A .docx file is a ZIP archive. That's part of why modern Office files are smaller and more resilient than the old binary ones.
Editable vs. fixed, one more time
The recurring theme: editable formats (DOCX, PPTX, XLSX, ODT, RTF, TXT) are for doing the work. Fixed formats (mainly PDF) are for finishing and sharing it. Most document headaches come from using the wrong camp — trying to edit a PDF, or emailing an editable file as if it were final.
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Converting between them
Because these formats do different jobs, moving between them is a normal part of the workflow: draft in Word, present in PowerPoint, finalise as PDF, and sometimes pull a PDF's pages out as images for a slide or a thumbnail. None of that should be scary — it's just choosing the right shape for the next step.
When you're not sure what software the other person has, PDF is the safe default — every phone, browser and computer can open it without extra apps.
