It began with a single tweet. A curious claim floated through social media: the maximum size for a PDF, supposedly, was 381 square kilometers — half the size of Germany or about as large as Iowa. For years, this peculiar fact had been passed along like digital folklore. But recently, Alex Chan, a software developer based in the UK, decided to test it.

What started as an idle curiosity ended with something astonishing: a PDF file larger than the entire observable universe.

The file format PDF — Portable Document Format — is everywhere. Invented by John Warnock of Adobe in 1993, it solved a vexing problem. Warnock, frustrated by computers that mangled graphics across different systems, wanted a way to ensure documents always appeared exactly the same, whether printed or displayed on-screen. On a napkin in a Silicon Valley restaurant, he sketched out his idea, calling it the “Camelot Project.” Within two years, it became the PDF, and Adobe Acrobat was born.

Initially, PDFs stumbled. They were bulky files, painfully slow to upload or download over the dial-up connections of the 1990s. Then, in 1996, the IRS chose PDF for tax submissions, marking the format’s first big break. Adoption accelerated again in 2008 when PDF became an open standard, freely available and universally accessible, according to Big Think.

Today, nearly everyone uses PDFs — from artists submitting portfolio samples, to astronauts reading manuals aboard spacecraft. The format’s popularity is staggering: in 2023 alone, roughly 2.5 trillion PDF documents were created. Most PDFs are ordinary, conforming to familiar sizes like A4 or the North American letter standard. But, as Chan discovered, the format itself imposes few practical limits.

Breaking the Limits of PDF Size

The supposed size limit — 381 kilometers per side — originates from technical constraints within Adobe’s Acrobat reader. Historically, Acrobat capped PDF dimensions at 15 million inches per side (381 km), roughly equal to the area of New York State. This fact circulated widely online, cemented by tweets and trivia maps.

Chan wasn’t convinced. “These posts are often accompanied by a ‘well, actually,’” he writes on his blog, explaining that the limitation was tied specifically to Adobe Acrobat, not to the PDF format itself. Digging deeper, he found that PDFs store page dimensions internally, measured in units — by default, 1/72 inch each. Starting from version 1.6 of the PDF format, released around 2007, these units could scale up dramatically.

Experimenting, Chan pushed past Acrobat’s boundaries, editing PDF files by hand using code — a process both painstaking and illuminating.

“It quickly became apparent why nobody writes PDFs by hand,” Chan notes. Still, this direct method allowed him to explore previously uncharted territory.

Acrobat refused to acknowledge anything larger than 381 kilometers. But Chan soon realized another common application, Apple’s Preview app, imposed no such constraints. Preview gladly accepted page dimensions larger than Germany. Chan expanded his PDF dimensions incrementally: soon, his document stretched a trillion units wide, about 352,778 kilometers — almost the exact distance from Earth to the Moon.

But he didn’t stop there.

Continuing his experiment, Chan eventually crafted a PDF with dimensions that defied imagination — approximately 37 trillion light-years square. “Admittedly, it’s mostly empty space,” he conceded, “but so is the universe.” To grasp this scale: the observable universe itself spans roughly 93 billion light-years in diameter. Chan’s PDF dwarfed it, proving definitively that the limitations ascribed to PDFs were not fundamental, but arbitrary.

“Please don’t try to print it,” Chan humorously warns. And perhaps that’s best: even the universe might not have enough ink.

If you’d like to play with the universe-sized PDF, you can get it here.