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Text of the page (random words):
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 and acroform fields the user password encrypts the file while the owner password does not instead relying on client software to respect these restrictions an owner password can easily be removed by software including some free online services 39 thus the use restrictions that a document author places on a pdf document are not secure and cannot be assured once the file is distributed this warning is displayed when applying such restrictions using adobe acrobat software to create or edit pdf files even without removing the password most freeware or open source pdf readers ignore the permission protections and allow the user to print or make copies of excerpts of the text as if the document were not limited by password protection 40 41 42 beginning with pdf 1 5 usage rights ur signatures are used to enable additional interactive features that are not available by default in a particular pdf viewer application the signature is used to validate that the permissions have been granted by a bona fide granting authority for example it can be used to allow a user 43 to save the pdf document along with a modified form or annotation data import form data files in fdf xfdf and text csv tsv formats export form data files in fdf and xfdf formats submit form data instantiate new pages from named page templates apply a digital signature to existing digital signature form field create delete modify copy import and export annotations for example adobe systems grants permissions to enable additional features in adobe reader using public key cryptography adobe reader verifies that the signature uses a certificate from an adobe authorized certificate authority any pdf application can use this same mechanism for its own purposes 43 under specific circumstances including non patched systems of the receiver the information the receiver of a digital signed document sees can be manipulated by the sender after the document has been signed by the signer 44 pades pdf advanced electronic signatures is a set of restrictions and extensions to pdf and iso 32000 1 45 making it suitable for advanced electronic signatures this is published by etsi as ts 102 778 46 file attachments edit pdf files can have file attachments which processors may access and open or save to a local filesystem 47 metadata edit pdf files can contain two types of metadata 2 the first is the document information dictionary a set of key value fields such as author title subject creation and update dates this is optional and is referenced from an info key in the trailer of the file a small set of fields is defined and can be extended with additional text values if required this method is deprecated in pdf 2 0 in pdf 1 4 support was added for metadata streams using the extensible metadata platform xmp to add xml standards based extensible metadata as used in other file formats pdf 2 0 allows metadata to be attached to any object in the document such as information about embedded illustrations fonts and images as well as the whole document attaching to the document catalog using an extensible schema pdf documents can also contain display settings including the page display layout and zoom level in a viewer preferences object adobe reader uses these settings to override the user s default settings when opening the document 48 the free adobe reader cannot remove these settings accessibility edit pdf files can be created specifically to be accessible to people with disabilities 49 50 51 52 53 pdf file formats in use as of 2014 update can include tags text equivalents captions audio descriptions and more some software can automatically produce tagged pdfs but this feature is not always enabled by default 54 55 leading screen readers including jaws window eyes hal and kurzweil 1000 and 3000 can read tagged pdfs 56 57 moreover tagged pdfs can be re flowed and magnified for readers with visual impairments adding tags to older pdfs and those that are generated from scanned documents can present some challenges one of the significant challenges with pdf accessibility is that pdf documents have three distinct views which depending on the document s creation can be inconsistent with each other the three views are i the physical view ii the tags view and iii the content view the physical view is displayed and printed what most people consider a pdf document the tags view is what screen readers and other assistive technologies use to deliver high quality navigation and reading experience to users with disabilities the content view is based on the physical order of objects within the pdf s content stream and may be displayed by software that does not fully support the tags view such as the reflow feature in adobe s reader pdf ua the international standard for accessible pdf based on iso 32000 1 was first published as iso 14289 1 in 2012 and establishes normative language for accessible pdf technology pdf ua addresses accessibility of the pdf format it brings the ideas behind wcag 2 0 to establish pdf specific accessibility rules 58 multimedia edit rich media pdf is a pdf file including interactive content that can be embedded or linked within the file it can contain images audio video content or buttons for example if the interactive pdf is a digital catalog for an e commerce business products can be listed on the pdf pages and can be added with images and links to the website and buttons to order directly from the document forms edit interactive forms is a mechanism to add forms to the pdf file format pdf currently supports two different methods for integrating ...
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