Why does my PDF print with unexpected margins or slightly cut-off edges?

Almost always an A4-versus-Letter mismatch. The two sizes differ by about half an inch in each direction, so a document laid out for one and printed on the other is either scaled down, leaving extra margin, or cropped at an edge.

Can pages in a single PDF have different sizes?

Yes. Page size is a per-page property, so a merged document can legitimately contain A4, Letter, and landscape pages together. Viewers handle this fine; printers are far less consistent about it.

Guide

PDF Page Sizes Explained: A4, Letter, and Why Your Pages Come Out Wrong

A document looks perfect on screen, then prints with a stripe of extra margin down one side, or with the last column of a table shaved off. Or you build a PDF from photos and every page turns out a different shape. Both trace back to the same thing: a PDF page has real physical dimensions, and they don't always match what you assumed.

PDF measures in points

PDF's internal unit is the point: exactly 1/72 of an inch, inherited from typography. Every page carries a box defining its size in points, and that box is what gives the page a physical identity rather than just a pixel count.

A4 — 595 × 842 points (210 × 297 mm)
US Letter — 612 × 792 points (8.5 × 11 in)
US Legal — 612 × 1008 points (8.5 × 14 in)
A3 — 842 × 1191 points
A5 — 420 × 595 points

Note what A4 and Letter do to each other: Letter is about 17 points wider and A4 is about 50 points taller. Neither fits inside the other. There is no orientation or setting that makes them the same, which is why the mismatch can only ever be resolved by scaling or cropping.

The A4 / Letter problem

A4 is standard essentially everywhere except the United States and Canada, so any document crossing that line hits the mismatch. Your printer driver resolves it one of three ways, and which one it picks explains what you're seeing:

Scale to fit — shrinks the page slightly to fit the paper. Everything is present but marginally smaller, with a wider margin on one or two sides. The commonest outcome, and usually harmless unless the document is dimension-critical.
Print actual size — no scaling, so content past the paper's edge is simply lost. This is where a table's last column disappears.
Fit to printable area — scales to the printer's mechanical margins, shrinking a little more than strictly needed.

If a printed PDF looks wrong, the print dialog's scaling option is the first place to look. It's also worth checking the document's own page size in your reader's properties panel before blaming the printer — a file exported for Letter will always be slightly awkward on A4 paper regardless of settings.

Why PDFs built from images have odd page sizes

An image has pixels, not inches. Turning one into a PDF page therefore requires a decision: how many pixels to a point? Different tools answer differently, which is why the same photos produce different-looking PDFs depending on what built them.

HerePDF's image to PDF tool sizes each page to match its source image, so the image fills the page exactly with no cropping and no padding. The upside is that nothing is ever cut off and no white borders appear. The consequence is that a 4000×3000 phone photo and a 1200×1600 scan produce pages of quite different physical sizes in the same document — which looks slightly untidy in a viewer, and can print inconsistently.

For most submissions this is fine; reviewers read on screen and portals check content rather than page geometry. When it does matter — a document that will definitely be printed, or one where a portal specifies A4 — the fix is upstream: resize the source images to a consistent aspect ratio and resolution before converting, so every page comes out the same shape. Scanning apps that output at a fixed page size handle this for you.

Mixed sizes in one document

Page size is a per-page property, so a single PDF can legitimately hold A4, Letter, and landscape pages side by side. This happens most often after merging documents from different sources, and nothing warns you because nothing is wrong.

Viewers cope well, resizing the window's page view as you scroll. Printers cope badly — many apply one paper setting to the whole job, so pages that don't match get scaled or clipped. If a merged document is destined for print, normalise page sizes in the source files before combining rather than trying to repair the merged result.

A note on the boxes

A page can carry several boxes, not just one. The MediaBox is the full page; the CropBox is the region actually displayed. When they differ, viewers show the CropBox — which is how a "cropped" PDF still contains everything outside the visible area, a point worth remembering if you ever crop a page hoping to remove something sensitive. It doesn't; the material is still in the file. That's covered properly in why hiding text in a PDF usually doesn't remove it.

Practical rules

Know your audience's paper. A4 for most of the world, Letter for North America — set it when you export, not afterwards.
When a print looks wrong, check the print dialog's scaling before anything else.
Normalise page sizes before merging if the result will be printed.
For image-built PDFs that must print predictably, make the source images consistent first.
Don't rely on cropping to hide anything.