The dimensions
A4 measures 210 by 297 millimeters, which is about 8.27 by 11.69 inches. US Letter measures 8.5 by 11 inches, or roughly 216 by 279 millimeters. A4 is narrower and taller; Letter is wider and shorter.
The shapes are close enough that a document designed for one usually prints on the other, but not without consequences. A Letter page printed on A4 loses a little width, and an A4 page printed on Letter loses height, so margins shift and content can clip at the edges.
Where each size is standard
A4 is the everyday sheet across most of the world, including Europe, Asia, Australia, and much of Africa and South America. Letter is standard in the United States and Canada. Printer paper shelves and office supply catalogs follow the same map.
A4 belongs to the ISO 216 system, where each size is half the previous one and the proportion never changes. Letter comes from an older office tradition instead. The practical effect is simple: check the destination before you export or print a form, because templates are built for one specific sheet.
Choosing a page size for a PDF
When you build a PDF from photos, the page size decides how each image is framed. Fit mode sizes the page around the image, while A4 and Letter place the image inside a standard sheet with a margin you control.
If the PDF will be printed in the United States or Canada, start with Letter. For almost anywhere else, use A4. Orientation handles the rest, and automatic orientation follows the shape of each image so a landscape photo is not turned on its side.