According to Adobe website, Portable Document Format (PDF) is a file format used to present and exchange documents reliably, independent of software, hardware or operating system. Invented by Adobe, PDF is now an open standard maintained by the International Organization for Standardization (ISO). PDFs can contain links and buttons, form fields, audio, video and business logic. They can also be signed electronically and are easily viewed using free Acrobat Reader DC software.
More clearly explained here https://goo.gl/3LCk7G
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