Compression can be described as the reduction in size of data in order to save space or transmission time. Compression can be carried out on particular data content or on entire transmission unit during a data transmission and this depends on number of factors. These factors can be like removing all extra space characters, inserting a single repeat character to indicate a string of repeated characters, and substituting smaller bit strings for frequently occurring characters in a particular content. Such kind of compression can reduce a text file to almost 50% of its original size. Compression is usually performed by a program that uses a formula or algorithm which determines how to compress or decompress the data. This algorithm is one of the crucial factors that determine the compression quality.
Compression can be either lossy (in this case some information is permanently lost) or lossless (here all information can be restored). In PDF compression, compression refers to image compressing. PDF formats are generally designed to compress information as much as possible. PDF is a page description language, like PostScript but it is simplified with restricted functionality. It has a better data structure and also very efficient compression algorithms to reduce the file size to about half the size of an equivalent PostScript file. Each compression method is suitable for certain specific color image. For example, by using JPEG compression, colour and grayscale images can be compressed by a factor of 10 or more. Similarly effective compression of monochrome images depends on the compression filter used and the properties of the image, but reductions of 50% to 12.5% are usually common.
Now a days there are many advanced softwares available in the market which can be used for PDF compression. In these, you can select the Compression, Quality, Image size options for Color, Grayscale and Monochrome images. Thus image compression means where you actually reduce the data in the image by lossless compression or by lossy compression and there are different algorithms for performing this operation. Various compression algorithms commonly used in PDF compression are Flate, JPEG, JPEG2000, CCITT G4 etc. types.