A file with the .pdf file extension is a Portable Document Format (PDF) file. PDFs are typically used to distribute read-only documents that preserve the layout of a page. They’re commonly used for documents like user manuals, eBooks, application forms, and scanned documents, to name just a few.
PDF was created by Adobe in the 1990s to achieve two things. The first is that people should be able to open the documents on any hardware or operating system, without needing to have the app used to create them—all you need is a PDF reader, and these days most web browsers fit the bill. The second is that wherever you open a PDF, the layout of the document should look the same.