The short answer
Send DOCX when the recipient needs to edit the file. Send PDF when the document is final and the layout should stay stable.
When PDF is better
PDF is better for resumes, contracts, invoices, reports, forms, and uploads where fonts, margins, page breaks, and spacing should not change.
When DOCX is better
DOCX is better for drafting, comments, collaboration, editing, and documents that still need revisions. Keep DOCX as the source and export PDF for final sharing.