PDF page size and orientation

A4 and Letter are nearly, but not quite, the same

A4 is 210 × 297 mm. US Letter is 216 × 279 mm. Letter is about 6 mm wider and 18 mm shorter — close enough that a document usually looks fine, and different enough that it goes wrong at the edges.

Print an A4 document on Letter paper and the bottom 18 mm has nowhere to go: a footer or a final line can be clipped, or the whole page is scaled down slightly and every margin shifts. Print Letter on A4 and you get an unexpected band of white at the bottom.

Two things make this harder to spot than it should be. A PDF can contain pages of different sizes — this is normal and legal, and it is what you get after merging documents from different sources. And a reader will happily display a mixed-size document at a consistent width, so it looks uniform on screen and prints inconsistently.

Page count and file info lists the size of each page, which is the quickest way to find out whether a document is mixed. Resize PDF pages brings them to one size.

Rotating is not resizing

These solve different problems and are often confused because both make a page "the other way round".

Rotation is a property of the page saying how it should be displayed: 0, 90, 180 or 270 degrees. The content underneath is untouched. This is what you want when a scan came in sideways, or when a landscape table was captured upright. Rotate PDF pages does this, and it is lossless — nothing is redrawn.

Resizing changes the page dimensions and scales the content to fit. This is what you want when an A4 document has to become Letter, or when a slide deck needs to print on paper. Resize PDF pages does this.

There is a third case that is neither: a page with the right dimensions but too much white space around the content. That is crop, which changes the visible area without scaling anything. Worth knowing that cropping usually only hides the material outside the new boundary rather than deleting it — if the point is to remove information rather than to tidy the view, redact is the tool, and how to redact a PDF permanently explains why the distinction matters.

Getting a clean print

• Check the pages before you print. Mixed sizes are the most common cause of a document that prints unevenly, and they are invisible on screen.
• Set the size in the file, not in the dialog. "Fit to page" in a print dialog rescales every time and differs between printers. Resizing the document once makes the result the same everywhere.
• Watch the margins. Most printers cannot print to the very edge. A document designed with 5 mm margins may lose content on a printer with a 10 mm unprintable border, whatever the page size says.
• Rotate before resizing. If a page is both sideways and the wrong size, fix the rotation first — resizing a rotated page scales it against the dimensions it is stored with, not the ones you see.

All three of these tools run in your browser, so a document being prepared for print is never uploaded. How browser PDF tools work covers the mechanism.