DOCX vs DOC: what changed?

A DOCX file is a ZIP archive. If you copy report.docx to report.zip, you can inspect folders such as word/document.xml, word/media/, and _rels/. Text, styles, numbering, comments, and embedded objects live in separate parts linked by relationship files.

A DOC file is a binary compound document from older Word versions. The content is stored in streams that are not meant to be edited by hand. That structure can preserve legacy Word behavior, but it makes corruption analysis and automated extraction more difficult.

DOC persists because old templates, line-of-business systems, and archived correspondence were created before Office Open XML became the normal exchange format. Some organizations also keep DOC output in macros or document generation libraries that have not been updated.

The age of the format matters because rendering depends on compatibility settings. A file may open in modern Word, but pagination can shift when it is saved as DOCX because Word no longer uses the exact same layout assumptions. That is not a failed conversion; it is Word interpreting old layout rules in a new package.

A plain DOCX file does not contain VBA macros. Macro-enabled Open XML documents use .docm. Legacy DOC files can contain macros, OLE objects, and old embedded data in ways that are harder to audit. Treat unknown DOC files from email as active documents, not just text containers.

When converting a DOC template, check fields, form controls, mail merge data, and embedded Excel charts. These objects may open correctly in Word but flatten, relink, or disappear in a headless conversion tool. If the document is legally sensitive, compare the Word-rendered PDF before and after conversion.

Use Word when exact compatibility with tracked changes, comments, and old fields matters. Use LibreOffice headless when you need server-side batch conversion and can tolerate small layout differences. A typical command is soffice --headless --convert-to docx --outdir out legacy.doc.

After conversion, open the DOCX and check fonts, page count, section breaks, tables, headers, and footnotes. Do not rely only on file size. A smaller DOCX may be cleaner, but it can also mean embedded objects or preview data were removed.

Do not convert a signed archival document just to modernize the extension. If the DOC is evidence, preserve the original and make a derived DOCX or PDF copy. Also avoid conversion when a receiving system explicitly requires legacy DOC, which still happens in some old form upload portals.

For editing and reuse, DOCX is normally the better working format. For long-term distribution, PDF or PDF/A may be a better final format because it freezes fonts and layout rather than exposing Word’s editable model.

When auditing old document stores, group files by purpose before converting. Templates, signed letters, generated reports, and one-off drafts have different risk levels. A batch DOC-to-DOCX job that is fine for editable drafts may be wrong for records that should remain exactly as received.

Keep one representative failure set. Include a macro document, a file with tracked changes, a document with old form fields, and a file with embedded spreadsheets. Those examples reveal whether your conversion path preserves the parts your organization actually uses.