When this is the right tool
Reach for this when someone else needs to edit what you have written. A PDF is the right answer for something final; a .docx is the right answer for a draft, a template or anything that will be marked up and sent back.
It is also the fastest way to get something out of a browser and into Word. Paste an article, a set of notes or a table from a web page and it arrives with proper heading styles already applied, rather than as a wall of text you have to restyle by hand.