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.
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.