The original imaging model of PDF was, like PostScript's, opaque: 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 view acceptably by older viewers, but files making extensive use of transparency could be viewed incorrectly by an older viewer.
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.
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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:[31]
In November 2019, researchers from Ruhr University Bochum and Hackmanit GmbH published attacks on digitally signed PDFs .[55] 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.[56] In 2021, they showed new so-called shadow attacks on PDFs that abuse the flexibility of features provided in the specification.[57] 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.[58][59]
Adobe Acrobat is one example of proprietary software that allows the user to annotate, highlight, and add notes to already created PDF files. One UNIX application available as free software (under the GNU General Public License) is PDFedit. The freeware Foxit Reader, available for Microsoft Windows, macOS and Linux, allows annotating documents. Tracker Software's PDF-XChange Viewer allows annotations and markups without restrictions in its freeware alternative. Apple's macOS's integrated PDF viewer, Preview, does also enable annotations as does the open-source software Skim, with the latter supporting interaction with LaTeX, SyncTeX, and PDFSync and integration with BibDesk reference management software. Freeware Qiqqa can create an annotation report that summarizes all the annotations and notes one has made across their library of PDFs. The Text Verification Tool exports differences in documents as annotations and markups.
This article provides an overview of the Microsoft Office Activation Wizard. Office includes an Activation Wizard. To fully use Office, you must activate Office. If you do not activate the product after you install it, the Office programs can be started only in reduced-functionality mode. In reduced-functionality mode, Office programs function more like viewers. In other words, you cannot save modifications to documents or create a new document. Additional functionality may be reduced also. No existing Office files are damaged while the product is in reduced-functionality mode.
What is reduced-functionality mode? In reduced-functionality mode, programs function similarly to a viewer. When running in reduced-functionality mode, many menu items are unavailable (dimmed), thereby blocking access to that functionality. Some of the limitations of reduced-functionality mode include the following limitations:
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