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Text of the page (random words):
n 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 data and pdf forms both formats today coexist in the pdf specification 43 59 60 61 acroforms also known as acrobat forms introduced in the pdf 1 2 format specification and included in all later pdf specifications xml forms architecture xfa forms introduced in the pdf 1 5 format specification adobe xfa forms are not compatible with acroforms 62 xfa was deprecated from pdf with pdf 2 0 acroforms were introduced in the pdf 1 2 format acroforms permit the uses of objects e g text boxes radio buttons etc and some code e g javascript alongside the standard pdf action types interactive forms acroforms support submitting resetting and importing data the submit action transmits the names and values of selected interactive form fields to a specified uniform resource locator url interactive form field names and values may be submitted in any of the following formats depending on the settings of the action s exportformat submitpdf and xfdf flags 43 html form format html 4 01 specification since pdf 1 5 html 2 0 since 1 2 forms data format fdf based on pdf uses the same syntax and has essentially the same file structure but is much simpler than pdf since the body of an fdf document consists of only one required object forms data format is defined in the pdf specification since pdf 1 2 the forms data format can be used when submitting form data to a server receiving the response and incorporating it into the interactive form it can also be used to export form data to stand alone files that can be imported back into the corresponding pdf interactive form fdf was originally defined in 1996 as part of iso 32000 2 2017 citation needed xml forms data format xfdf external xml forms data format specification version 2 0 supported since pdf 1 5 it replaced the xml form submission format defined in pdf 1 4 the xml version of forms data format but the xfdf implements only a subset of fdf containing forms and annotations some entries in the fdf dictionary do not have xfdf equivalents such as the status encoding javascript page s keys embeddedfdfs differences and target in addition xfdf does not allow the spawning or addition of new pages based on the given data as can be done when using an fdf file the xfdf specification is referenced but not included in pdf 1 5 specification and in later versions it is described separately in xml forms data format specification 63 the pdf 1 4 specification allowed form submissions in xml format but this was replaced by submissions in xfdf format in the pdf 1 5 specification xfdf conforms to the xml standard xfdf can be used in the same way as fdf e g form data is submitted to a server modifications are made then sent back and the new form data is imported in an interactive form it can also be used to export form data to stand alone files that can be imported back into the corresponding pdf interactive form as of august 2019 xfdf 3 0 is an iso iec standard under the formal name iso 19444 1 2019 document management xml forms data format part 1 use of iso 32000 2 xfdf 3 0 64 this standard is a normative reference of iso 32000 2 pdf the entire document can be submitted rather than individual fields and values as was defined in pdf 1 4 acroforms can keep form field values in external stand alone files containing key value pairs the external files may use forms data format fdf and xml forms data format xfdf files 65 63 66 the usage rights ur signatures define rights for import form data files in fdf xfdf and text csv tsv formats and export form data files in fdf and xfdf formats 43 in pdf 1 5 adobe systems introduced a proprietary format for forms adobe xml forms architecture xfa adobe xfa forms are not compatible with iso 32000 s acroforms feature and most pdf processors do not handle xfa content the xfa specification is referenced from iso 32000 1 pdf 1 7 as an external proprietary specification and was entirely deprecated from pdf with iso 32000 2 pdf 2 0 licensing edit anyone may create applications that can read and write pdf files without having to pay royalties to adobe systems adobe holds patents to pdf but licenses them for royalty free use in developing software complying with its pdf specification 67 security edit see also adobe acrobat security changes to content edit in november 2019 researchers from ruhr university bochum and hackmanit gmbh published attacks on digitally signed pdfs 68 they showed how to change the visible content in a signed pdf without invalidating the signature in 21 of 22 desktop pdf viewers and 6 of 8 online validation services by abusing implementation flaws at the same conference they additionally showed how to exfiltrate the plaintext of encrypted content in pdfs 69 in 2021 they showed new so called shadow attacks on pdfs that abuse the flexibility of features provided in the specification 70 an overview of security issues in pdfs regarding denial of service information disclosure data manipulation and arbitrary code execution attacks was presented by jens müller 71 72 malware vulnerability edit some popular pdf readers have a history of security vulnerabilities that allows pdf files that have been infected with viruses trojans and other malware to inflict damage such pdf files can have hidden javascript code that might exploit vulnerabilities in a pdf reader hidden objects executed when the file that hides them is opened and less commonly a malicious pdf can launch malware 73 pdf attachments carrying viruses were first discovered in 2001 the virus named outlook pdfworm or peachy uses microsoft outlook to send itself as an attached adobe pdf file it was activated with adobe acrobat but not with acrobat reader 74 over the years several vulnerabilities have been discovered in various versions of adobe reader 75 which prompted the company to issue security fixes vulnerabilities have been discovered in other pdf readers as well one aggravating factor is that a pdf reader can be configured to start automatically if a web page has an embedded pdf file providing a vector for attack if a malicious web page contains an infected pdf file that takes advantage of a vulnerability in the pdf reader the system may be compromised even if the browser is secure some of these vulnerabilities are a result of badly written pdf readers mishandling javascript embedded in the pdf file disabling javascript execution in the pdf reader can help mitigate such future exploits although it does not protect against exploits in other parts of the pdf viewing software some security experts say that javascript is not essential for a pdf reader and that the security benefit that comes from disabling javascript outweighs any compatibility issues caused 76 one way of avoiding pdf file exploits is to have a local or web service convert files to another format before viewing on march 30 2010 security researcher didier stevens reported an adobe reader and foxit reader exploit that runs a malicious executable if the user allows it to launch when asked 77 zip bomb edit see also zip bomb pdf streams can have nested filters which allows one to craft a 5 kilobyte file that unpacks to 1 petabyte in ram this can be used to cause a denial of service with implementations that do not guard against this like it was the case with pypdf s cve 2025 55197 78 79 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