DOCX is the natural default today
DOCX is the newer format and is better supported by modern office software. If you are creating a new document and have no legacy requirement, DOCX is usually the simpler choice for editing, sharing, and conversion.
When does old DOC still appear?
You may still receive archived documents or templates stored as DOC. There is no need to rewrite them solely because the extension is old. Open the file, review fonts and tables, then convert it when you need a fixed PDF for distribution.
Check fonts and layout before PDF
The biggest formatting surprises often come from fonts, margins, tables, and floating objects rather than the extension itself. Review page size before conversion and inspect pages with complex layouts after the PDF is created.
Keep the editable source
PDF is excellent for delivery and printing, but it should not replace your editable source. Keep the original DOCX or DOC if future edits are likely, and use the PDF as the sharing copy.
Do not treat Save As as the only compatibility test
An application may open an old DOC and save it as DOCX without an obvious warning, while legacy fields, macros, or layout features can still behave differently. For an important document, compare several pages after saving and keep the old copy until you are satisfied.
Choose the format for the person doing the next step
If someone else will edit the file, ask what software or system they use rather than assuming every office suite behaves identically. If the document is final and only needs to be read, PDF often reduces display differences. The best format is the one that fits the next step in the workflow.