chm The CHM file format and its associated .chm filename extension, is a proprietary file format developed by Microsoft. Its full name is compiled HTML Help. CHM was released to the public in 1997 as a bundle with Microsoft Windows 98 and was primarily designed to facilitate documentation on the Windows platform, particularly help files for software applications that run on the Microsoft Windows platform. Essentially, a CHM file is a collection of HTML files packed as a single compressed file. Though the format is proprietary, it was successfully reverse engineered by the late 90's to early 2000s allowing the development of open source readers that are capable of opening files with the .chm extension. The format was succeeded by the LIT format.
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.