The author of the web content encountered a claim stating that there is a maximum size for a PDF document, and they decided to investigate its validity. They discovered that this claim originated from the limitation of a particular PDF reader app, rather than a limitation of PDF itself. Adobe Acrobat imposes a limit of 15 million by 15 million inches, which is approximately 381 kilometers. Through experimentation and manual editing of the PDF structure, the author was able to create PDFs with large dimensions. They even created a PDF that Preview.app claimed to be larger than the entire universe.
https://alexwlchan.net/2024/big-pdf/