Contents in PDF

A PDF file is a combination of text, bitmap graphics, and vector graphics. The basic types of content in a PDF are: text stored as content streams, vector graphics for illustrations and designs that consist of shapes and lines, raster graphics for photographs and other types of image.

A PDF document can also support links (inside document or web page), forms, JavaScript (initially available as plugin for Acrobat 3.0), or other types that can be handled using plug-ins. Furthermore, any files can be embedded into a PDF file as attachments.

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.

A PDF file size can be reduced by replacing contained images, optimizing embedded fonts, removing unused data or stream compression.