A4 and US Letter look similar on screen but have different proportions. A4 is 210 × 297 mm (about 8.27 × 11.69 inches). Letter is 8.5 × 11 inches (215.9 × 279.4 mm). Letter is wider; A4 is taller.
That small difference becomes visible in margins, page breaks and print scaling.
Choose for the destination
A4 is standard in most countries, while Letter is common in the United States and Canada. Use the size required by the recipient, printer, filing system or official form rather than choosing from habit.
Understand Fit, Fill and Stretch
Fit preserves aspect ratio and keeps all content visible, possibly adding margins. Fill preserves aspect ratio but may crop edges. Stretch forces content to the new dimensions and can distort text and graphics. Fit is the safest default for documents.
Check before printing
Use Resize PDF only when a uniform output size is necessary. Inspect pages near every edge, then print at Actual Size if dimensions must be exact or Fit if the printer’s non-printable margins would otherwise clip content. Avoid converting back and forth repeatedly.