The document is final - contracts, invoices, CVs, certificates
Layout must be identical for every reader
You are submitting through a portal that specifies PDF
The recipient should not casually edit it
Send Word when
The recipient must edit, comment or fill it in
You are collaborating on a draft
Accessibility tools need reflowable text
You genuinely do not know the recipient's software - .docx opens almost everywhere
Converting between them
Word to PDF locks the layout for sharing. PDF to Word re-flows the text for editing - expect to tidy the formatting afterwards on complex layouts. Convert at the last step: edit in Word, share in PDF.
A practical habit
Keep the .docx as your master copy and generate the PDF fresh each time you need to send it. That way edits stay easy and the shared version is always up to date.