PDF Page Sizes: A4, Letter, and More
PDFs can use any page size, but in practice almost every document uses one of a handful of standards. The two you will meet most often are A4 and Letter — and they are not the same.
ISO 216
The A-series (A4, A3, A5)
The A-series is defined by ISO 216 and used across most of the world. Every A-series page has the same aspect ratio of 1 : √2 (about 1 : 1.414). That ratio has a useful property: folding a page in half along its long edge gives the next smaller size with the same proportions.
- A4 — 210 × 297 mm. The default page size in Europe, Asia, Africa, and most of the world. Used for letters, forms, and reports.
- A3 — 297 × 420 mm. Twice A4. Used for posters, diagrams, and large drawings.
- A5 — 148 × 210 mm. Half of A4. Used for booklets and notepads.
US & Canada
North American sizes
The United States, Canada, and a few other places use a different set of sizes that are not based on ISO 216:
- Letter — 8.5 × 11 in (216 × 279 mm). The default for business documents in North America.
- Legal — 8.5 × 14 in (216 × 356 mm). Used for contracts and legal filings.
- Tabloid / Ledger — 11 × 17 in (279 × 432 mm). Used for larger layouts and spreads.
A4 and Letter are close in size but not identical. A4 is slightly taller and narrower than Letter. A document designed for one will have slightly different margins when printed on the other — which is why "fit to page" scaling exists in print dialogs.
Beyond A4 and Letter
Other sizes you may meet
Two more families appear in specialized contexts. TheB-series (B5, B4) sits between A-series sizes and is used for posters, books, and envelopes. TheC-series (C4, C5, C6) is designed for envelopes that hold A-series sheets — a C5 envelope holds a folded A5 sheet, and a C4 envelope holds an unfolded A4 sheet. Neither is common in everyday office PDFs.
Presentation slides often use a landscape page sized to match a screen aspect ratio — 10 × 7.5 in for 4:3, or 13.33 × 7.5 in for 16:9 widescreen. These are not paper standards but are frequent in PDFs exported from slide software.
In practice
Why this matters for PDFs
A PDF stores its page size explicitly, so a document created on A4 stays A4 no matter where it is opened. That is one of the format's main advantages over word-processor files, which can reflow depending on the local default page size. If you share a PDF internationally, the recipient sees exactly the page you designed.
When converting images or documents to PDF, pick the page size that matches the intended use. A photo meant for screen viewing does not need A4 margins; a form meant for printing should match the paper in the recipient's region.
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