Paper size comparison
A4 vs US Letter size
A4 and US Letter are close enough to be confused but different enough to change margins and pagination.
What changes when you scale
Fit-to-page scaling can shrink text and alter margins. A document designed close to the edge may also be clipped if automatic scaling is disabled.
Choose the target before layout
Use A4 for most international workflows and Letter for common US and Canadian office workflows. Ask the recipient or printer when the destination is unclear.
Check the exported PDF
Application page settings and the final PDF can disagree. Inspect the exported file rather than relying only on the document setup screen.