A page-size tool can map a PDF to A4, Letter, or custom dimensions, but it needs a rule for content: fit proportionally, stretch, preserve size with margins, or crop. Stretching distorts graphics, while fitting can make small text smaller.
Test a page with edge-to-edge content and one with generous margins. Inspect the new media box, printed output, text legibility, and page orientation. Mixed-size documents may need per-page decisions rather than one global setting. Keep the original PDF and verify the result in the printer or viewer you actually use.