PDF - The Portable Document Format is a platform-independent file format developed by Adobe Systems. 

To open and view PDF documents you need a reader such as Adobe Acrobat Reader or PDF-X-Change Viewer. PDF files can be created with most common word processing programs (for example Libre Office and MS Word), often using the "Save as..." or "Export file" function.

There are several PDF standards. PDF/A is suitable for long-term storage. Whether a PDF file is PDF/A or not may be looked up in the file properties. More security is provided by validation, e.g. with Adobe Acrobat Professional. 

An existing PDF file can only be edited to a limited extent because the format was designed for exchanging finished files. However, with most viewers or readers it is possible to insert notes into a document.

Advantages:

- independent of the used platform 

- Suitability for long-term storage (PDF/A)

- the software to display is free of charge

Disadvantages:

- To modify a PDF file you need special software