The Portable Document Format (PDF) is a file format developed by Adobe in the 1990s to present documents, including text formatting and images, in a manner independent of application software, hardware, and operating systems. Based on the PostScript language, each PDF file encapsulates a complete description of a fixed-layout flat document, including the text, fonts, vector graphics, raster images and other information needed to display it. PDF was standardized as an open format, ISO 32000, in 2008, and no longer requires any royalties for its implementation.
Today, PDF files may contain a variety of content besides flat text and graphics including logical structuring elements, interactive elements such as annotations and form-fields, layers, rich media (including video content) and three dimensional objects using U3D or PRC, and various other data formats. The PDF specification also provides for encryption and digital signatures, file attachments and metadata to enable workflows requiring these features.
A PDF file is often a combination of vector graphics, text, and bitmap graphics. The basic types of content in a PDF are:
- Text stored as content streams (i.e., not encoded in plain text)
- Vector graphics for illustrations and designs that consist of shapes and lines
- Raster graphics for photographs and other types of image
- Multimedia objects in the document
PDF 1.6 supports interactive 3D documents embedded in the PDF – 3D drawings can be embedded using U3D or PRC and various other data formats.
Two PDF files that look similar on a computer screen may be of very different sizes. For example, a high resolution raster image takes more space than a low resolution one. Typically higher resolution is needed for printing documents than for displaying them on screen. Other things that may increase the size of a file is embedding full fonts, especially for Asiatic scripts, and storing text as graphics.