DOCX and PDF look similar when you open them, but they solve opposite problems. Knowing which is which saves you from sending the wrong format at the wrong moment.
DOCX is a living document
A .docx file is built for editing. Under the hood it's a small archive of XML describing paragraphs, styles, tables, and images, designed so Word, Google Docs, or LibreOffice can keep changing it. The cost of that flexibility: the same file can render differently on different machines. If your reader is missing a font or opens it in another app, spacing shifts and a table can slide onto the next page.
PDF is a snapshot
A PDF freezes every letter and image to a fixed spot on a fixed page. It looks identical on a phone, a laptop, or a printer, anywhere in the world. That's why résumés, contracts, invoices, and official forms are almost always requested as PDF: the sender can be sure the layout survives the trip.
The rule of thumb
- Still writing or revising? Stay in .docx.
- Sending it for someone to read, print, or sign? Convert to PDF.
- Need the recipient to edit it back? Send the .docx (or both).
Convert too early and you'll re-export after every edit. Convert too late and you risk sending an editable file where formatting — or tracked changes and comments you forgot to clear — can leak to the reader.
Simple test: if you'd be unhappy for the reader to change a single word, it should be a PDF.