A PDF can have pages at any size. Many PDFs created in the United States use US Letter (8.5 × 11 inches); most PDFs from Europe and the rest of the world use A4 (210 × 297 mm). When you print a US Letter PDF on A4 paper — or submit it to a system that expects A4 — the page proportions do not match, and the output has unexpected margins or slight scaling.
Resizing a PDF standardises all pages to a single size. The content is scaled proportionally to fit inside the new page dimensions, and whitespace is added where the proportions do not match exactly. Nothing is cropped.
A4 vs US Letter
A4 (ISO 216) is 210 × 297 mm (595 × 842 points at 72 DPI). It is slightly taller and narrower than US Letter. A4 is used in virtually every country outside the United States and Canada — the default in printers, word processors, and document systems across Europe, Asia, Africa, and South America.
US Letter (ANSI A) is 8.5 × 11 inches (612 × 792 points). It is shorter and wider than A4. It is the default in the US and Canada, and the default page size in Microsoft Word when installed in those regions.
The difference is small enough that a mixed-size document often prints acceptably on the wrong paper, but it can cause alignment issues in forms, PDFs that are paginated for a specific size, and submissions to systems that validate page dimensions.
Landscape pages
Landscape pages stay landscape. A landscape page is resized to landscape A4 (297 × 210 mm) or landscape Letter (11 × 8.5 in). The rotation is preserved; only the size changes.
File handling
Resize PDF runs entirely in your browser using pdf-lib. The file never reaches a server. The resize is a transform applied to each page's content stream — no rendering, no rasterization. Text stays selectable, fonts stay embedded, and the output is structurally identical to the input, just at the new page size.