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 before combining it, or run the merged file through the Resize tool (see the measurements below).

What we measured: the sizes real PDFs have

We read the page boxes of 126,395 pages in 4,636 real documents (every PDF that opens, in a fixed collection of 2009-era US government PDFs) and worked out the size each page is given to a reader: its CropBox clipped to its MediaBox, turned by its rotation setting, using the same geometry HerePDF's own tools use. Then we ran the Resize tool to A4 and to Letter over 2,889 of the files and read them back.

Letter dominates this collection, and mixing sizes inside one file is uncommon. 109,756 pages (87%) are US Letter and 2,464 (1.9%) are A4; 13,730 (11%) match none of Letter, A4, Legal, A3, A5 or Tabloid. In 3,914 of the 4,636 files every page is Letter, and in 79 every page is A4. Of the 4,195 files with more than one page, 3,743 (89%) have one paper, one way up, on every page; 204 (4.9%) hold one paper both portrait and landscape; and 248 (5.9%) hold more than one paper size, up to 82 different sizes in one file. Only 5 of those put Letter and A4 together, so in this collection the A4 and Letter clash happens between a document and the paper it meets, not inside one file.

The size a viewer shows is often not the sheet the file was made for. 13,730 pages match no named paper, and 5,447 of those sit under a CropBox; on 5,031 of them the MediaBox underneath is a standard sheet (37% of all the odd sizes). In other words, many "odd" sizes are a standard sheet with its margins trimmed from view, and the sheet is still in the file. The two commonest odd sizes are 538 x 718 points (1,454 pages) and 432 x 648 (970). A rotation setting does the same job for orientation: 10,311 pages (8.2%) in 440 files carry one, and 10,214 of them are stored one way up and shown the other, so a page whose MediaBox is portrait appears landscape.

Resize reached the exact size on every page we tried. 2,468 of the files are Letter on every page. Resize to A4 (Fit) gave A4 on 31,920 of their 31,920 pages with no page added or lost, scaling each page down by less than a tenth. The 50 files that are A4 on every page went the other way to Letter on 677 of 677 pages, also shrinking each page by less than a tenth. Across all 2,889 files in the test, A4 was reached on 37,682 of 37,682 pages and Letter on 37,682 of 37,682, and no file changed its page count. Fit does not cut off anything you can see: it scales the content to fit inside the new page and the spare room shows as margin. The converted Letter files came out a median 96.6% of their original size, because HerePDF rewrites the file as it saves it, and the largest was 123.0%.

The limits: these are old government documents, not yours, and a collection from the US over-represents Letter. A page is named when both edges are within 3 points of a standard size; any other size is compared after rounding to the point. 204 of the 4,840 files are not counted: 197 were encrypted and the rest could not be read. The file names are not published, only counts; the data and the method are there to check against.

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, and cropping a PDF: what stays hidden in the file.

Practical rules

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