PDF or DOCX is a question of life stage, not of file format. Documents start as DOCX and finish as PDF — that is the rule of thumb.
Here is when each one is right, and what to do at the boundary.
Use DOCX when
You are still drafting. You want others to edit it. You are collaborating with comments and tracked changes. You expect to keep updating it. DOCX is the working file.
Use PDF when
The document is finished. You want it to look identical everywhere. You need a signature on it. You are archiving it. You are sending it to someone who should not edit it. PDF is the published file.
The boundary
Crossing from DOCX to PDF is what 'Save as PDF' does in Word. Crossing back is what convert-pdf-to-word does in Flint. Going from PDF back to DOCX always loses something — fonts, layout, sometimes images.
Best for…
DOCX for working drafts. PDF for finished, signed, archival or distributed documents.
FAQ
Can I edit a PDF without converting back?
Yes — edit-pdf on Flint changes text in place without round-tripping through Word.
Why does PDF-to-Word lose formatting?
PDF stores positioned text and shapes, not a document structure. Reconstruction is approximation.
Which is smaller?
Depends. PDFs with embedded fonts can be larger; DOCX with embedded images can be larger.
DOCX while you write, PDF when you ship. Cross the boundary cleanly with convert-pdf.