PDF is a file format used for creating and sharing electronic documents. The acronym stands for Portable Document Format. The format was created by Adobe Systems in 1991 for the purpose of creating digital documents that could be accessed and shared on any operating system or platform.

The PDF format is widely used for its ability to preserve the integrity of the content and the visual formatting of the document, regardless of the software or hardware being used to view it. It is also used because of its security features, which allow users to protect their documents by requiring a password or setting restrictions on who can view or edit the file.

There are several versions of the PDF format, each with different features and capabilities. The most widely used PDF format is PDF 1.7, which was introduced in 2006. This version includes features such as transparency, layers, and enhanced security options.

There are also several subsets of the PDF format, which are designed for specific purposes. For example, PDF/A is a subset of the PDF format that is designed for archiving purposes, while PDF/X is a subset that is used for creating print-ready files.

In addition to these standard versions of the PDF format, there are also several proprietary formats that have been created by software companies. These formats are designed to work with specific software applications, and are not always compatible with other software programs or platforms.