Portable Document Format, a file format for representing the layout and appearance of documents on a page independent of the layout software, computer operating system, etc. It is a file format used to present and exchange documents independently of software, hardware or operating systems. An open standard maintained by the International Organisation for Standardisation.  Originally a proprietary format of Adobe Systems, PDF has been an open format since 2008. Data in PDF files is not machine-readable; see structured data.

Source: ODH and  https://acrobat.adobe.com/be/en/products/about-adobe-pdf.html