Which is bigger, A4 or Letter?

Neither, exactly. A4 is taller by about 18 mm and Letter is wider by about 6 mm. A4 has slightly more area, but you cannot fit one inside the other in either orientation.

Can I change a PDF's page size without reprinting it?

You can crop, which trims the page box and discards what falls outside it, or scale, which resizes the content to fit a new box. Both change the file. Neither recovers content that was already cut off by a previous conversion.

Why does my PDF print with margins I did not set?

Almost always the print dialog's scaling. "Fit to printable area" shrinks the page to clear the printer's own unprintable border, adding a margin all round. Choosing 100% or "actual size" prints at the intended dimensions, at the risk of clipping if the paper is smaller.

a4 vs letter pdf size

A4, Letter, and Legal Page Sizes

A4 and Letter are close enough to look interchangeable and different enough to break a layout. A4 is 6 mm narrower and 18 mm taller, which is why documents from abroad print with clipped edges or an unexpected blank strip.

The measurements

A4 is the ISO 216 standard used almost everywhere outside North America. US Letter and Legal are the North American standards. Legal shares Letter's width and adds three inches of height, which is why legal-size documents feel long rather than wide.

A4: 210 × 297 mm — 8.27 × 11.69 in — 595 × 842 pt
US Letter: 216 × 279 mm — 8.5 × 11 in — 612 × 792 pt
US Legal: 216 × 356 mm — 8.5 × 14 in — 612 × 1008 pt
A3: 297 × 420 mm — exactly two A4 pages side by side

Why the small difference causes trouble

Against Letter, A4 is about 6 mm narrower and 18 mm taller. Print an A4 document on Letter paper at full size and the bottom 18 mm falls off the page — which is where page numbers and footers live. Print Letter on A4 and you get an 18 mm blank strip at the bottom and 6 mm of content risking the right margin.

Most print dialogs offer to scale to fit, which solves the clipping and introduces a subtler problem: everything is now 96% of its intended size. For a report nobody notices. For anything where the printed dimensions matter — forms, labels, technical drawings, anything to be signed and filed — it matters a great deal.

Points, pixels, and why PDFs measure in neither

PDF measures in points, at 72 to the inch, which is why A4 comes out as 595 × 842 and Letter as 612 × 792. These are the numbers you will see in file properties and in any tool that reports page dimensions.

There is no DPI in a PDF page itself. A PDF page has a physical size, and images placed on it have their own resolution — an image at 300 DPI simply means it has 300 pixels for every inch it occupies. This is why "what resolution is this PDF" has no answer, while "what resolution are the scans inside it" does.

Handling mixed sizes

Merging files of different sizes keeps every page at its own dimensions. That is correct behaviour rather than a bug — nothing has been distorted — but the printed stack will have uneven edges, and duplex printing on mixed sizes is awkward.

If you need one consistent size, decide which way to go before you print. Cropping trims to a chosen box and loses whatever falls outside it. Scaling keeps all the content and changes its physical size. For a document that will be read on screen, neither is worth doing; for one that will be filed on paper, pick deliberately.

One practical rule: if you are sending a document abroad, A4 is the safer default. It is the standard in more countries than Letter, and a Letter document printed on A4 loses nothing — it just sits slightly high on the sheet.