Every time you save a document you make a choice with long-term consequences: who will be able to open it, with which programs, and whether it will still be readable in ten years. The distinction between open and closed formats helps you make that choice consciously, instead of accepting the default option every time.
What «open format» means
A format is open when its specification is public, documented and usable by anyone to write programs that read and produce it, without depending on a single vendor. Often it is also standardised by a body such as ISO.
The practical consequence is that the document is not tied to one program: if a piece of software disappears or changes its licence, others can keep opening the files.
What ODT and DOCX look like inside
It may come as a surprise, but an .odt or .docx file is a ZIP archive. Try copying one, changing the extension to .zip and opening it: you will find folders and XML files.
document.docx (ZIP archive)
├── [Content_Types].xml
├── word/
│ ├── document.xml ← the text
│ ├── styles.xml ← the styles
│ └── media/ ← the images
└── docProps/ ← the metadata
An ODT has a similar structure, with content.xml, styles.xml and meta.xml. This detail has two concrete uses: recovering the original images from a document by extracting the media folder, and saving part of the text from a damaged file that the program can no longer open.
The main formats
| Format | Extensions | Nature | Strengths | Limits |
|---|---|---|---|---|
| OpenDocument (ODF) | .odt, .ods, .odp | ISO standard, designed to be neutral | Default in LibreOffice, well documented | Some features may render differently in Word |
| Office Open XML (OOXML) | .docx, .xlsx, .pptx | ISO standard, derived from Microsoft formats | Extremely widespread, excellent rendering in Word | Very large specification, with variants |
| RTF | .rtf | Public specification, no longer developed | Opens almost anywhere | No modern features, heavy files |
| Plain text | .txt, .md | Universal | Readable forever, perfect for Git | No page layout |
| ISO standard | Fixed appearance, ideal for delivery | Not designed to be edited |
A note on OOXML: the standard includes a Strict variant and a Transitional one, which keeps compatibility elements with the old formats. Word normally saves in Transitional. For most people the difference is invisible, but it explains why a DOCX does not always behave the same way in different programs.
Formats to avoid for archiving
The old binary formats, such as .doc, .xls and .ppt, were poorly documented for years and are harder to interpret correctly. If you have an archive of files like these, it is worth converting them gradually: LibreOffice can do it in bulk from the command line too.
soffice --headless --convert-to odt *.doc
The same command, with --convert-to docx or --convert-to pdf, produces other formats. Keep the originals until you have spot-checked that the conversion is correct.
How to choose
The choice depends on what the file has to do:
- Collaboration with Word users: DOCX, because it reduces layout shifts.
- Working in LibreOffice or a requirement for neutral open formats: ODT.
- Delivering a final version: PDF, possibly PDF/A for archiving.
- Notes, technical documentation, texts under version control: Markdown or plain text.
- Exchange with very old or unknown programs: RTF as a last resort.
In my work I keep sources in an open, editable format and produce the PDF only as a delivery copy. That way the document stays workable even if the tool changes.
Reducing differences between programs
Opening the same document in Word and in LibreOffice can give slightly different results. The measures that reduce the problem are few and concrete:
- Use styles instead of direct formatting.
- Choose fonts available on all systems or embed them when the program allows it.
- Avoid text boxes and absolutely positioned objects, which are the first things to move.
- Prefer simple tables, without complex merged cells.
- Check the document in the recipient's program before delivering it.
In short
- An open format has a public specification and does not depend on a single vendor.
- ODT and DOCX are ZIP archives of XML files: handy for recovering images and text.
- ODF and OOXML are both ISO standards; old binary .doc files should be converted.
- The choice depends on use: DOCX for Word, ODT for neutral formats, PDF for delivery, text for documentation.
- Styles, common fonts and simple layout reduce differences between programs.
