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Text of the page (random words):
marily of objects of which there are nine types 19 boolean values representing true or false real numbers integers strings enclosed within parentheses or represented as hexadecimal within single angle brackets strings may contain 8 bit characters names starting with a forward slash arrays ordered collections of objects enclosed within square brackets dictionaries collections of objects indexed by names enclosed within double angle brackets streams usually containing large amounts of optionally compressed binary data preceded by a dictionary and enclosed between the stream and endstream keywords the null object comments using 8 bit characters prefixed with the percent sign may be inserted objects may be either direct embedded in another object or indirect indirect objects are numbered with an object number and a generation number and defined between the obj and endobj keywords if residing in the document root beginning with pdf version 1 5 indirect objects except other streams may also be located in special streams known as object streams marked type objstm this technique enables non stream objects to have standard stream filters applied to them reduces the size of files that have large numbers of small indirect objects and is especially useful for tagged pdf object streams do not support specifying an object s generation number other than 0 an index table also called the cross reference table is located near the end of the file and gives the byte offset of each indirect object from the start of the file 29 this design allows for efficient random access to the objects in the file and also allows for small changes to be made without rewriting the entire file incremental update before pdf version 1 5 the table would always be in a special ascii format be marked with the xref keyword and follow the main body composed of indirect objects version 1 5 introduced optional cross reference streams which have the form of a standard stream object possibly with filters applied such a stream may be used instead of the ascii cross reference table and contains the offsets and other information in binary format the format is flexible in that it allows for integer width specification using the w array so that for example a document not exceeding 64 kib in size may dedicate only 2 bytes for object offsets to ensure backward compatibility a hybrid reference pdf file may include both traditional cross reference tables and cross reference streams allowing older pdf processors to read the file while still taking advantage of the new features introduced in version 1 5 30 at the end of a pdf file is a footer containing the startxref keyword followed by an offset to the start of the cross reference table starting with the xref keyword or the cross reference stream object followed by the eof end of file marker if a cross reference stream is not being used the footer is preceded by the trailer keyword followed by a dictionary containing information that would otherwise be contained in the cross reference stream object s dictionary a reference to the root object of the tree structure also known as the catalog root the count of indirect objects in the cross reference table size other optional information within each page there are one or multiple content streams that describe the text vector and images being drawn on the page the content stream is stack based similar to postscript 31 there are two layouts to the pdf files non linearized not optimized and linearized optimized non linearized pdf files can be smaller than their linear counterparts though they are slower to access because portions of the data required to assemble pages of the document are scattered throughout the pdf file linearized pdf files also called optimized or web optimized pdf files are constructed in a manner that enables them to be read in a web browser plugin without waiting for the entire file to download since all objects required for the first page to display are optimally organized at the start of the file 32 pdf files may be optimized using adobe acrobat software or qpdf page dimensions are not limited by the format itself however adobe acrobat imposes a limit of 15 million by 15 million inches or 225 000 000 000 000 square inches 145 161 km 2 56 047 sq mi 2 1129 imaging model edit the basic design of how graphics are represented in pdf is very similar to that of postscript except for the use of transparency which was added in pdf 1 4 pdf graphics use a device independent cartesian coordinate system to describe the surface of a page a pdf page description can use a matrix to scale rotate or skew graphical elements a key concept in pdf is that of the graphics state which is a collection of graphical parameters that may be changed saved and restored by a page description pdf has as of version 2 0 25 graphics state properties of which some of the most important are the current transformation matrix ctm which determines the coordinate system the clipping path the color space the alpha constant which is a key component of transparency black point compensation control introduced in pdf 2 0 vector graphics edit as in postscript vector graphics in pdf are constructed with paths paths are usually composed of lines and cubic bézier curves but can also be constructed from the outlines of text unlike postscript pdf does not allow a single path to mix text outlines with lines and curves paths can be stroked filled filled then stroked or used for clipping strokes and fills can use any color set in the graphics state including patterns pdf supports several types of patterns the simplest is the tiling pattern in which a piece of artwork is specified to be drawn repeatedly this may be a colored tiling pattern with the colors specified in the pattern object or an uncolored tiling pattern which defers color specification to the time the pattern is drawn beginning with pdf 1 3 there is also a shading pattern which draws continuously varying colors there are seven types of shading patterns of which the simplest are the axial shading type 2 and radial shading type 3 raster images edit raster images in pdf called image xobjects are represented by dictionaries with an associated stream the dictionary describes the properties of the image and the stream contains the image data less commonly small raster images may be embedded directly in a page description as an inline image images are typically filtered for compression purposes image filters supported in pdf include the following general purpose filters ascii85decode a filter used to put the stream into 7 bit ascii asciihexdecode similar to ascii85decode but less compact flatedecode a commonly used filter based on the deflate algorithm defined in rfc 1951 deflate is also used in the gzip png and zip file formats among others introduced in pdf 1 2 it can use one of two groups of predictor functions for more compact zlib deflate compression predictor 2 from the tiff 6 0 specification and predictors filters from the png specification rfc 2083 lzwdecode a filter based on lzw compression it can use one of two groups of predictor functions for more compact lzw compression predictor 2 from the tiff 6 0 specification and predictors filters from the png specification runlengthdecode a simple compression method for streams with repetitive data using the run length encoding algorithm and the image specific filters dctdecode a lossy filter based on the jpeg standard ccittfaxdecode a lossless bi level black white filter based on the group 3 or group 4 ccitt itu t fax compression standard defined in itu t t 4 and t 6 jbig2decode a lossy or lossless bi level black white filter based on the jbig2 standard introduced in pdf 1 4 and jpxdecode a lossy or lossless filter based on the jpeg 2000 standard introduced in pdf 1 5 normally all image content in a pdf is embedded in the file but pdf allows image data to be stored in external files by the use of external streams or alternate images standardized subsets of pdf including pdf a and pdf x prohibit these features text edit text in pdf is represented by text elements in page content streams a text element specifies that characters should be drawn at certain positions the characters are specified using the encoding of a selected font resource a font object in pdf is a description of a digital typeface it may either describe the characteristics of a typeface or it may include an embedded font file the latter case is called an embedded font while the former is called an unembedded font the font files that may be embedded are based on widely used standard digital font formats type 1 and its compressed variant cff truetype and beginning with pdf 1 6 opentype additionally pdf supports the type 3 variant in which the components of the font are described by pdf graphic operators fourteen typefaces known as the standard 14 fonts or base fourteen fonts 33 have a special significance in pdf documents times v3 in regular italic bold and bold italic courier in regular oblique bold and bold oblique helvetica v3 in regular oblique bold and bold oblique symbol zapf dingbats these fonts or suitable substitute fonts with the same metrics should be available in most pdf readers but they are not guaranteed to be available in the reader and may only display correctly if the system has them installed 34 fonts may be substituted if they are not embedded in a pdf within text strings characters are shown using character codes integers that map to glyphs in the current font using an encoding there are several predefined encodings including winansi macroman and many encodings for east asian languages and a font can have its own built in encoding although the winansi and macroman encodings are derived from the historical properties of the windows and macintosh operating systems fonts using these encodings work equally well on any platform pdf can specify a predefined encoding to use the font s built in encoding or provide a lookup table of differences to a predefined or built in encoding not recommended with truetype fonts 2 the encoding mechanisms in pdf were designed for type 1 fonts and the rules for applying them to truetype fonts are complex for large fonts or fonts with non standard glyphs the special encodings identity h for horizontal writing and identity v for vertical are used with such fonts it is necessary to provide a tounicode table if semantic information about the characters is to be preserved a text document which is scanned to pdf without the text being recognised by optical character recognition ocr is an image with no fonts or text properties transparency edit the original imaging model of pdf was opaque similar to postscript where each object drawn on the page completely replaced anything previously marked in the same location in pdf 1 4 the imaging model was extended to allow transparency when transparency is used new objects interact with previously marked objects to produce blending effects the addition of transparency to pdf was done by means of new extensions that were designed to be ignored in products written to pdf 1 3 and earlier specifications as a result files that use a small amount of transparency might be viewed acceptably by older viewers but files making extensive use of transparency could be viewed incorrectly by an older viewer the transparency extensions are based on the key concepts of transparency groups blending modes shape and alpha the model is closely aligned with the features of adobe illustrator version 9 the blend modes were based on those used by adobe photoshop at the time when the pdf 1 4 specification was published the formulas for calculating blend modes were kept secret by adobe they have since been published 35 the concept of a transparency group in pdf specification is independent of existing notions of group or layer in applications such as adobe illustrator those groupings reflect logical relationships among objects that are meaningful when editing those objects but they are not part of the imaging model additional features edit logical structure and accessibility edit see also pdf a 1 and pdf ua a tagged pdf see clause 14 8 in iso 32000 includes document structure and semantics information to enable reliable text extraction and accessibility 36 technically speaking tagged pdf is a stylized use of the format that builds on the logical structure framework introduced in pdf 1 3 tagged pdf defines a set of standard structure types and attributes that allow page content text graphics and images to be extracted and reused for other purposes 37 tagged pdf is not required in situations where a pdf file is intended only for print since the feature is optional and since the rules for tagged pdf were relatively vague in iso 32000 1 support for tagged pdf among consuming devices including assistive technology at is uneven as of 2021 38 iso 32000 2 however includes an improved discussion of tagged pdf which is anticipated to facilitate further adoption an iso standardized subset of pdf specifically targeted at accessibility pdf ua was first published in 2012 optional content groups layers edit with the introduction of pdf version 1 5 2003 came the concept of layers layers more formally known as optional content groups ocgs refer to sections of content in a pdf document that can be selectively viewed or hidden by document authors or viewers this capability is useful in cad drawings layered artwork maps multi language documents etc basically it consists of an optional content properties dictionary added to the document root this dictionary contains an array of optional content groups ocgs each describing a set of information and each of which may be individually displayed or suppressed plus a set of optional content configuration dictionaries which give the status displayed or suppressed of the given ocgs encryption and signatures edit a pdf file may be encrypted for security in which case a password is needed to view or edit the contents 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 pdf files may also contain embedded drm restrictions that provide further controls that limit copying editing or printing these restrictions depend on the reader software to obey them so the security they provide is limited the standard security provided by pdf consists of two different methods and two different passwords a user password which encrypts the file and prevents opening and an owner password which specifies operations that should be restricted even when the document is decrypted which can include modifying printing or copying text and graphics out of the document or adding or modifying text notes 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