Why this operation exists
Headers and footers give a report its filename, issue date, confidentiality label or page position without editing every page.
What remains your responsibility
A new line can collide with existing artwork when the source already uses the edge of the page. That boundary is why the source should remain available throughout review.
Prepare process inspect
The working sequence is short: prepare the uploaded PDF; process it as Add Header and Footer to PDF; inspect a downloadable PDF file; then test it in the receiving application. Inspect portrait, landscape, first, last and unusually cropped pages before issuing the result.
Use the result wisely
Apply labels after page order is final so counters and section references remain trustworthy.