PDF (Portable Document Format) is an open standard for document exchange. The file format created by Adobe Systems in 1993 is used for representing two-dimensional documents in a manner independent of the application software, hardware, and operating system. Each PDF file encapsulates a complete description of a fixed-layout 2D document that includes the text, fonts, images, and 2D vector graphics which compose the documents. Today, 3D drawings can be embedded in PDF documents with Acrobat 3D using U3D or PRC and various other data formats. Originally a proprietary format, PDF was officially released as an open standard on July 1, 2008, and published by the International Organization for Standardization as ISO/IEC 32000-1:2008.
History:
Adobe PDF format is 18 years old when it was created in 1993 with the first version 1.0. During these years, this popular file format increased a lot and came through several versions, below lists the details for the PDF's development.
- - (1993) – PDF 1.0 / Acrobat 1.0
- - (1994) – PDF 1.1 / Acrobat 2.0
- - (1996) – PDF 1.2 / Acrobat 3.0
- - (1999) – PDF 1.3 / Acrobat 4.0
- - (2001) – PDF 1.4 / Acrobat 5.0
- - (2003) – PDF 1.5 / Acrobat 6.0
- - (2005) – PDF 1.6 / Acrobat 7.0
- - (2006) – PDF 1.7 / Acrobat 8.0
- - (2008) – PDF 1.7, Adobe Extension Level 3 / Acrobat 9.0
- - (2009) – PDF 1.7, Adobe Extension Level 5 / Acrobat 9.1
- - The latest release of Adobe Acrobat Reader is Acrobat X, a new way of viewing PDF files thanks to the new reader's improved user interface.
File Structure:
A PDF file is composed of four main parts. They are one-line header, body, cross-reference table and trailer.
Header of the PDF:
The first line of the PDF specifies the version of a PDF file format. These headers are the topmost portion of a document. For example, %PDF-1.4.
Body of PDF: The body of a PDF file consists of objects that compose the contents of the document. These objects include image data, fonts, annotations, text streams, hyperlinks, bookmarks as well as two-dimensional vector graphics.
Cross-Reference Table: The cross-reference table consists of links to all the objects or elements in a file. When a person updates a PDF file, it automatically gets updated in the cross-reference table.
Trailer:The trailer contains links to cross-reference table. It ends up with %%EOF to identify the end of a PDF file.
Interactive Elements:
PDF files may contain interactive elements such as annotations and form fields.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 PDF specification:
- - AcroForms (also known as Acrobat forms), introduced in the PDF 1.2 format specification and included in all later PDF specifications.
- - Adobe XML Forms Architecture(XFA) forms, introduced in the PDF 1.5 format specification. The XFA specification is not included in the PDF specification, it is only referenced as an optional feature. Adobe XFA Forms are not compatible with AcroForms.
Security and Signature:
A PDF file may be encrypted for security, or digitally signed for authentication. The standard security provided by Acrobat PDF consists of two different methods and two different passwords, "user password" and "owner password". A PDF document may be protected by password to open ('user' password) and the document may also specify operations that should be restricted even when the document is decrypted: printing, copying text and graphics out of the document, modifying the document, or adding or modifying text notes and AcroForm fields (using 'owner' password).
Subsets:
There are 4 kinds of subsets are being standardized under ISO for several constituencies:
- - PDF/X for the printing and graphic arts as ISO 15930 (working in ISO TC130)
- - PDF/A for archiving in corporate/government/library/etc environments as ISO 19005 (work done in ISO TC171)
- - PDF/E for exchange of engineering drawings (work done in ISO TC171)
- - PDF/UA for universally accessible PDF files
By the way, a PDF/H variant (PDF for Healthcare) is being developed. However, it may consist more of a set of "best practices" than of a specific format or subset.
