What is a .pdf file?

PDF is a universal document format that preserves layout, fonts, and formatting across every device and operating system. Every PDF you've ever read was originally something else. A Word document, a spreadsheet, a design file, a scanned receipt — at some point, someone decided it needed to look the same everywhere, so they turned it into a PDF. That's the entire point of the format: pixel-perfect layout preservation no matter what software created it. Adobe invented it in 1993, then handed it to ISO as an open standard. Today PDFs carry invoices, tax forms, academic papers, legal contracts, architectural blueprints, and the boarding pass on your phone. The format supports fillable forms, digital signatures, encryption, embedded fonts, and multimedia. It is, for better or worse, the paper of the internet.