txt From the 70's to 2007 the open source .txt filename extension format commonly referred to plain text documents encoded using the ASCII character set. To support internationalization and localization, .txt text documents are today text files encoded using the UTF-8 or UTF-16LE standard which is a superset of the ASCII character set. Text documents of type .txt typically have minimal formatting for example no support for bold or italic characters or support for bullet points etc. This allows .txt documents to use minimum storage space and be platform independent as long as the operating system supports the underlying encoding character set used to create the .txt document. On windows .txt file support has existed since 1985 when Windows 1.0 was released and since then has been mostly associated with the notepad application on Microsoft Windows.
pdf The .pdf file format extension was developed by Adobe systems in 1993 as a means of presenting documents in a consistent manner across different platforms, hardware, operating systems, and applications. The format was not released as an open source document format until 2008, though minimal proprietary technologies still controlled by Adobe Systems exist in the format. Every .pdf document carries with it the necessary meta information required to properly reconstruct the text, fonts, and graphics used to produce the document. This ensures that documents will be viewed in exactly the same way the author intended regardless of the device used to open the document. From conception to date, Adobe Systems has continued to support the format adding features with every iteration of the standard including hardened algorithms for document encryption and privacy. Today, many freely available readers exist allowing you to open and view .pdf documents and create or convert other file formats such as .jpeg and .doc into .pdf documents.