PDF Page Sizes and Paper Formats
Page size is the one property of a PDF that almost every later decision depends on, and it is routinely misread. This guide covers how PDF measures a page, the formats you will actually meet, and why a single document can contain pages of several different sizes.
A page is measured in points, not pixels
A PDF page has no resolution. It is described in points, where one point is exactly 1/72 of an inch, and everything on the page is placed in that coordinate system. This is why a PDF looks equally sharp at any zoom level: there are no pixels until something decides to draw it.
That also means the question "how many pixels is this page?" has no answer until you choose one. A page is a physical size; pixels appear only when a renderer picks a resolution. An A4 page is 8.3 by 11.7 inches whether you view it on a phone or send it to a printing press.
Points are a typographic unit, which is why they appear in odd-looking decimals. A4 is 210 by 297 millimetres, which works out to 595.28 by 841.89 points. Letter, defined in inches, lands on the round numbers 612 by 792 instead.
A series, B series and Letter
The ISO A series is built on a single rule: each size has the same aspect ratio, and halving the long side gives the next size down. A0 is one square metre, A1 is half of it, and so on to A4 at 210 by 297 millimetres. The constant ratio is why scaling an A4 layout up to A3 never leaves empty strips.
Letter, at 8.5 by 11 inches, is the North American default and is not part of that system. It is slightly wider and noticeably shorter than A4, so a document laid out for one will either crop or leave a margin on the other. The difference is small enough to miss on screen and large enough to ruin a print run.
The B series sits between A sizes and comes in two incompatible variants: ISO B5 is 176 by 250 millimetres, while the JIS B5 used in Japan is 182 by 257. If a Japanese document and an international template disagree about B5, this is usually why.
Why one document can have several page sizes
Nothing in the PDF format requires pages to match. Each page carries its own size, so a report assembled from several sources can easily mix A4 body pages with a Letter cover and an A3 foldout. Viewers hide this well, because they scale each page to the window.
It becomes visible the moment the document is printed, bound, or converted to images. Mixed sizes produce images with different pixel dimensions, which is disconcerting when they are meant to be a uniform set of thumbnails.
Landscape pages add a second wrinkle. A landscape page is usually stored as a portrait page with a rotation flag rather than as a genuinely wider page, so the stored dimensions and the dimensions you see can be swapped. Well-behaved renderers apply the rotation; code that reads the raw values sometimes does not.
MediaBox, CropBox, TrimBox and bleed
A PDF page can define up to five rectangles. The MediaBox is the full sheet as it exists in the file. The CropBox is the region a viewer displays, which is usually the same but can be smaller. The TrimBox is the finished page after cutting, and the BleedBox is the slightly larger area that includes artwork running past the trim line.
Bleed exists because cutting is not perfectly accurate. If a coloured background is meant to reach the edge of a finished page, it must be drawn a few millimetres beyond it, so a slight misalignment of the blade still leaves no white sliver. Japanese printers typically ask for 3 millimetres; the North American convention is an eighth of an inch.
This matters when converting to images because most renderers use the CropBox, which on a print-ready file includes the bleed and the crop marks. The exported image will then be larger than the finished page and have registration marks around the content. If that is not what you want, the file needs to be trimmed before conversion rather than after.
Turning page size into pixels
Once you know the page size in points, the pixel count follows from the resolution you choose. Since a point is 1/72 inch, rendering at 72 DPI gives one pixel per point, and any other resolution scales from there. An A4 page is 595 by 842 pixels at 72 DPI, 1190 by 1684 at 144 DPI, and 2894 by 4093 at 350 DPI.
In this converter, the scale setting is that multiplier. Scale 1x is 72 DPI, 2x is 144, and so on, so the pixel dimensions of your output are simply the page size in points multiplied by the scale. Knowing the page size therefore tells you exactly what you are going to get before you press convert.