A practical guide to PDF page sizes

PDF pages are measured in points, where 72 points is one inch. A4 is 595 by 842 points. US Letter is 612 by 792, wider and shorter. That mismatch is behind a surprising share of printing complaints.

Why the difference causes trouble

Print an A4 document on Letter paper and the page is too tall to fit, so the bottom is cut or the whole page is scaled down, leaving uneven margins. Print Letter on A4 and you get white bands on two edges. Neither is a fault in the file; they are just different shapes.

If a document is going to be printed in a specific country, set it to that country's standard size before sending. A4 covers most of the world; Letter covers North America.

Boxes within boxes

A PDF page has several defined rectangles, and they are not the same thing:

• MediaBox, the full physical page
• CropBox, what a viewer displays
• TrimBox, the finished page after cutting, used in commercial print
• BleedBox, the area including print bleed

Cropping in most tools adjusts the CropBox. That changes what you see, and it does not remove anything. The content outside the box is still in the file and can be recovered by resetting the box. This matters if you are cropping to hide something rather than to tidy margins.

How mixed sizes get in

Usually by merging. A report built from a Word export, a scanned signature page and a spreadsheet printed to PDF will contain at least two page sizes, often three. It looks fine on screen, where each page is displayed at its own size, and then prints inconsistently.

Standardising to a single size at merge time avoids the problem entirely.

Images are a special case

An image has pixels, not points, so placing one on a page requires a decision about scale. A 3000-pixel-wide photograph can be a small sharp image or a large soft one. Fitting inside the margins is almost always what people want; filling the page crops the edges; original size can produce a page far larger than any printer will take.

If you are building a PDF from photographs, pick a standard page size rather than matching each image, unless the file is only ever going to be viewed on screen.

Points, and why 72 matters

PDF measures in points, at 72 to the inch. That number comes from typography rather than computing, and it is why page dimensions look arbitrary in software that reports them: A4 is 595.28 by 841.89 points, because it is defined in millimetres and converts untidily.

Useful conversions to keep in mind: 72 points is one inch, 28.35 points is one centimetre, and a typical body text size of 11 points is a shade under four millimetres tall.

Margins are worth thinking about in the same units. A 20-point margin is about 7 mm, which is tight. A comfortable document margin is 50 to 70 points.

Mixed sizes and where they come from

Almost always from merging. A bundle assembled from a Word export, a scanned signature page and a spreadsheet printout will hold at least two page sizes and often three.

It looks fine on screen, because a viewer displays each page at its own size. It prints inconsistently, because the printer has to decide what to do with each one, and it usually decides differently for each.

Standardising to a single size at the moment of merging avoids the entire problem, and takes one setting.

Scaling, fitting and filling

When content moves between sizes there are three behaviours, and picking the wrong one is the usual cause of complaints.

Fit scales the content until it sits inside the new page, preserving proportions and leaving margins. This is almost always what you want for documents.

Fill scales until the content covers the page, cropping whatever overflows. Right for photographs used as backgrounds, wrong for anything with words near the edge.

Original size places content unscaled, which produces clipping if the new page is smaller.

Printing across regions

If a document is going to be printed in North America, set it to Letter. Anywhere else, A4. The difference is small, Letter is wider and shorter, but it is enough to shift a footer off the page or leave uneven white bands.

For documents that will be printed in both, the pragmatic answer is to design with generous margins so neither conversion clips anything important.

Checking before you send

Look at the page size of the first and last page, not just the first. Bundles frequently change size partway through, and the last page is where the appendix everyone forgot about lives.

Checking sizes across a whole document

Look at the first page, the last page, and the page after each section boundary rather than assuming the document is uniform. Bundles change size partway through more often than not, and the appendix is where it usually happens.

Images, points and pixels

An image has pixels; a page has points. Placing one on the other requires deciding a scale, and that decision is where photographs come out either soft or enormous.

A useful rule: divide the image's pixel width by the width in inches you want it to occupy. If the result is above about 200, you have detail to spare. Below 150, it will look soft in print, though it may be perfectly acceptable on screen.