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What “bigger than the universe” means here
The claim concerns the page’s coordinate system, not the PDF’s storage size or physical content. Chan’s document contained very little data; its page dictionary described an enormous blank canvas. In Chan’s experiment, macOS Preview reportedly showed dimensions of approximately 37 trillion light-years square. The careful wording is “Preview reported a page that large,” not that a physical object exceeded the universe.
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The comparison is also informal. The observable universe is commonly estimated at about 93 billion light-years in diameter, while “37 trillion light-years square” can be read as a square’s side length or as loose area language. The reliable point is that the viewer displayed a nominal page dimension vastly beyond that astronomical comparison.
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Where the 381-kilometre limit came from
Internet discussions often say that a PDF page cannot exceed 381 km by 381 km. Chan traced that figure to limits associated with Adobe Acrobat, rather than to a universal physical maximum imposed by every PDF implementation.
| Value | Meaning | Result in the cited Acrobat context |
|---|---|---|
| 1/72 inch | Default PDF user-space unit | Base scale for page coordinates |
| 14,400 × 14,400 units | Acrobat’s cited maximum /MediaBox |
200 × 200 inches before additional scaling |
| 75,000 | Acrobat’s cited maximum /UserUnit in Acrobat 7.0 documentation |
Multiplies each side to 15,000,000 inches |
| 15,000,000 inches per side | Combined cited limits | Approximately 381 × 381 kilometres |
Those figures describe what particular Acrobat versions supported or reported. An application’s supported range is not automatically the same as the PDF format’s theoretical range.
How PDF page dimensions are encoded
/MediaBox: the page boundary
The /MediaBox array defines the page’s rectangular coordinate limits. With the default unit of 1/72 inch:
/MediaBox [0 0 300 300]describes a page about 4.17 by 4.17 inches. Changing the values to:
/MediaBox [0 0 600 600]describes a page about 8.33 by 8.33 inches.
/UserUnit: scaling the coordinate unit
/UserUnit scales the default user-space unit. For example:
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/MediaBox [0 0 14400 14400]
/UserUnit 2makes the nominal page 400 by 400 inches instead of 200 by 200 inches. At /UserUnit 75000, the cited Acrobat calculation reaches 15,000,000 inches per side, or about 381 kilometres.
A fragment from Chan’s experiment looks like this:
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<<
/Type /Page
/Parent 3 0 R
/MediaBox [0 0 14400 14400]
/UserUnit 75000
/Contents 1 0 R
>>This is not a complete PDF. A usable file also needs its other objects, byte offsets, cross-reference table, trailer, startxref pointer and %%EOF marker.
How Chan pushed beyond Acrobat’s practical range
Testing Acrobat’s ceiling
Chan hand-edited and generated minimal PDFs while investigating the recurring 381-km claim. When values exceeded Acrobat’s supported range, Acrobat continued to report the maximum it could handle or effectively ignored the larger values. That behavior made the number look like a format limit even though it was an application limit.
Using macOS Preview instead
Preview behaved differently in Chan’s tests. It reportedly ignored /UserUnit, used the default 1/72-inch unit, and did not impose Acrobat’s same upper bound on /MediaBox. Chan could therefore enlarge the coordinate bounds directly.
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A /MediaBox width of 1,000,000,000,000 user units corresponds to about 13.89 billion inches, or approximately 352,778 kilometres—roughly the Earth–Moon distance—at the default scale. Increasing the value further led Preview to display the reported result of about 37 trillion light-years square.
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“PDF size” can describe several unrelated things:
- File size: the number of bytes stored on disk.
- Page dimensions: the nominal width and height encoded in page metadata.
- Page area: width multiplied by height.
- Content extent: where marks, text or images actually appear.
- Rendered output: what a viewer, printer or driver can display or produce.
Chan’s PDF was mostly empty space. A vector document can describe a huge canvas without storing pixels for every part of it, just as a map can cover a large geographic area while containing little detail. A file with marks distributed throughout that entire coordinate range would present very different storage, indexing and rendering problems.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why PDF viewers disagree
The result sits at the intersection of three layers:
- PDF’s data model: page objects, drawing commands, metadata and cross-reference information.
- Application implementation: viewers choose numeric ranges, scaling behavior and error handling.
- Output hardware: printers and drivers impose their own sheet-size, memory and rasterization limits.
Consequently, the same file might appear capped in Acrobat, cropped in another viewer, trigger an out-of-range warning, open as an ordinary page if metadata is ignored, or fail to render at all. Preview accepting a value does not mean every PDF reader supports unlimited dimensions.
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What would happen if you tried to print it?
“Please don’t try to print it” is mostly a joke, but printing would be impractical and likely incompatible. A print workflow could:
- reject the page as outside the driver’s supported range;
- scale it down to a normal sheet;
- crop the page;
- tile it across many pages;
- attempt an unwieldy rasterization; or
- hang, fail or report an error after exhausting software resources.
There is no evidence that merely sending the file to a printer would physically destroy the hardware. The realistic risk is failed or badly scaled output.
The PDF structure behind the experiment
Chan constructed simple files by hand rather than relying entirely on a PDF library. A minimal PDF commonly includes:
- a header such as
%PDF-1.6; - numbered indirect objects for pages and content;
- a cross-reference table (
xref); - a trailer with document metadata;
- a
startxrefpointer; and - the
%%EOFmarker.
The cross-reference table records where objects begin, allowing a reader to locate them efficiently. On April 16, 2025, Chan corrected a handwritten example: xref 0 4 should have been xref 0 5 because the table contained five entries. The correction is a reminder that hand-authored PDF internals are exacting even when the visible document is nearly blank.
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Chan did not make a file physically larger than the universe, and did not prove that PDFs have unlimited practical dimensions. They created a tiny document with astronomical page coordinates, then found a viewer willing to display those coordinates. The experiment shows why “size” in digital documents needs a qualifier: bytes, canvas dimensions, content, rendering and printing are different measurements.
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