Portable Document format is essentially a page description language, which also acts as a container for many objects. It has restricted functionality but lighter when compared to Post Script. PDF files containing only text information are mostly compressed by itself, due to its design and structure. But when several additional information and objects are imported, the size tends to bloat. Therefore action to compress PDF file must be taken. PDF compression normally refers to image compression, as the size is a direct consequence of the total image size. PDF files not only have an efficient structure by itself, but they also incorporate additional high level compression algorithms. The Algorithms used in to compress PDF file are: LZW, JPEG and JPEG2000, FLATE, JBIG2, CCITT, RLE (Run Length Encoding).
These algorithms can be divided into two categories mainly: Lossy and Lossless compression. Lossless algorithms do not result in a loss of data in its final output. They are essentially used for Text compression. Lossy algorithms is just the opposite, and result in loss of some data. Such algorithms can be used for images or sound files but not for text or program data.
Every method to compress PDF file is suitable for specific color images. For instance,JPEG compression can results in a compression ratio of more than 10 for grayscale images. 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 common (or 5% to 2% for JBIG2 compression of an image of a page full of text). LZW or Flate compression of the content streams describing all other text and graphics in the document results in compression ratios of approximately 50%.
Additional software to compress PDF file are also available. These are either commercially distributed or freely distributed under an open source license. Files can be automatically compressed by using these software. Some of them also have a batch file processing feature i.e. is several files can be queued for compression and left unattended. This makes compression of large volume of files even easier and quicker. For increased security, password protected archives are also one of their features.