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PDFtk (short for PDF Toolkit) is a toolkit for manipulating Portable Document Format (PDF) documents. [ 3 ] [ 4 ] It runs on Linux , Windows and macOS . [ 5 ] It comes in three versions: PDFtk Server ( open-source command-line tool ), PDFtk Free ( freeware ) and PDFtk Pro ( proprietary paid ). [ 2 ]
PDF 2.0 defines 256-bit AES encryption as the standard for PDF 2.0 files. The PDF Reference also defines ways that third parties can define their own encryption systems for PDF. PDF files may be digitally signed, to provide secure authentication; complete details on implementing digital signatures in PDF are provided in ISO 32000-2.
The dictionary compressor finds matches using sophisticated dictionary data structures, and produces a stream of literal symbols and phrase references, which is encoded one bit at a time by the range encoder: many encodings are possible, and a dynamic programming algorithm is used to select an optimal one under certain approximations.
Thus, a representation that compresses the storage size of a file from 10 MB to 2 MB yields a space saving of 1 - 2/10 = 0.8, often notated as a percentage, 80%. For signals of indefinite size, such as streaming audio and video, the compression ratio is defined in terms of uncompressed and compressed data rates instead of data sizes:
Introduction to Compression Theory (PDF), Wiley, archived (PDF) from the original on 2007-09-28; EBU subjective listening tests on low-bitrate audio codecs; Audio Archiving Guide: Music Formats (Guide for helping a user pick out the right codec) MPEG 1&2 video compression intro (pdf format) at the Wayback Machine (archived September 28, 2007)
Most lossless compression programs do two things in sequence: the first step generates a statistical model for the input data, and the second step uses this model to map input data to bit sequences in such a way that "probable" (i.e. frequently encountered) data will produce shorter output than "improbable" data.
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