What are the differences between the subsets in the PDF standard and what are they used for?

Ricardo Lee

2022-08-04 11:28

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PDF stands for Portable Document Format, which is a programmatic document format. All displayed content is drawn through corresponding operators. Vector graphics are used in PDF files, so no matter how many times the file is enlarged, the text and patterns drawn with vector graphics will not be distorted. PDF has the advantages of cross-platform, high compression, suitable for screen reading and network transmission, file protection, electronic review, and high printing quality.


PDF is a common document format created by Adobe Corporation, originally used to facilitate the dissemination of digital information on various types of computers within the company. At the Comdex Expo in 1992, Adobe released PDF 1.0 to the outside world for the first time.

However, with the prosperity of cross-media publishing, especially since the current e-book carrier is gradually transitioning to mobile media terminals, many discomforts of the PDF format have gradually emerged. First of all, this format is specially designed for printing on standard paper, and it cannot automatically adjust the page width to fit the display on different size screens; Secondly, as a formatted document, PDF lacks a lot of document logic structure information, and cannot easily implement data analysis such as image and text separation to meet increasingly complex application requirements.

To solve such a problem, a series of standards that everyone abides by is needed to constrain and regulate PDF files. Each different PDF standard format corresponds to different needs. For example, the PDF/A standard is used for long-term archiving of electronic documents, the PDF/X standard is used for color printing, and the PDF/E standard is used for the interactive exchange of engineering documents. The following is a brief description of the relevant requirements of the PDF/A, PDF/X, and PDF/E standards based on the introduction of the relevant websites.

1. PDF/A (suitable for long-term archiving)


In September 2005, the International Organization for Standardization (ISO) approved a new PDF/A standard for archiving electronic documents. PDF/A is a variant of PDF according to the ISO 19005-1 standard. It "provides a tool for reproducing an electronic document in a way that retains its appearance over a long period, regardless of the tools and systems with which the document was created, stored or produced."

The PDF/A standard is often used for the long-term archiving of electronic documents. The difference from ordinary PDF files is that it restricts some functions that are not suitable for long-term preservation. In fact, PDF/A is a restricted subset of PDF, omitting features in PDF that are not suitable for long-term archiving, such as different system fonts, digital signatures, and JavaScript. This is similar to PDF/X, another subset of PDF for printing. In addition, the PDF/A standard sets the necessary conditions for software products that read PDF/A files. A standards-compliant reader must follow specified rules, including following color management guidelines, rendering with embedded fonts, etc.

At present, the PDF/A format document has become the standard stipulated by most governments and industry associations, and many government agencies including UNESCO and the governments of the United States, Germany, the United Kingdom, Brazil, and other countries have adopted PDF/A as an electronic file Standard format for saving. In December 2005, the ISO organization issued the PDF/A-1 standard, and in 2011 and 2012, PDF/A-2 and PDF/A-3 were successively introduced. The main differences are as follows:

PDF/A-1a: Also known as Level A Conformance, fully compliant with the ISO-19005-1 standard. This version includes tabs so that text can be extracted and viewed from a variety of devices, including handheld devices. PDF/A-1a implements the preservation of the logical structure of the document and the preservation of text in natural reading order.

PDF/A-1b: Also known as Level B Conformance. This category counts as the lowest level of compliance for PDF/A. This level guarantees that the document can be displayed and viewed on a computer screen, but the legality of the text is not guaranteed. PDF/A-1b enables the correct display of text and additional content. The main difference between PDF/A-1a and PDF/A-1b is how each handles text extraction.

PDF/A-2: This is a newer addition to the PDF/A standard and is still in the specification process of a "Technical Committee". Basically, PDF/A-2 will handle some of the newer features attached to PDF references such as electronic signatures.

PDF/A-3: PDF/A-3 is based on the ISO standard 32000-1 and offers support for embedded files.

2. PDF/X (for printing)


Although PDF documents support a variety of digital page elements, if they contain music files or video files, the PDF printing workflow can be problematic. Therefore, the PDF/X format and its usage are defined to ensure that only the data elements that need to be printed are included in PDF/X-compliant PDF documents. PDF/X was introduced as a PDF-based print workflow standard.

PDF/X, like PDF/A, is a subset of PDF. The purpose of PDF/X is to provide designers, drafters, engineers, and graphic artists with an electronic file format that can be printed correctly by any service provider. This format is ideal for the transfer of print-ready files by most network companies; here, the sender and receiver of print-ready information are not strongly correlated. In addition to providing a solid delivery format for print jobs, PDF/X has other benefits, including a file viewer, better compression (smaller file size), and support for spot color printing colors that recognize printing conditions ( such as which file is ready), and more. However, like PDF/A, the benefits of PDF/X come with some compromises. Features such as transparency, encryption, and JBIG2 compression are prohibited in PDF/X.

The PDF/X standards are divided into three categories:

PDF/X-1a: This standard is ideal for file senders who want to maintain the most control over their printing jobs. It supports blind send exchange of information and conforms to the general requirements used in many parts of the world. This type of document itself contains all the elements that need to be printed. Color data can only be in CMYK mode and spot colors, all images, and fonts must be built into the file, and transparent layers are prohibited. Is a subset of the PDF/X-3 standard.

PDF/X-3: This allows the use of device-independent color forms, as well as device-dependent data that is not flagged. The most important difference between PDF/X-3 and PDF/X-1a is the fact that PDF/X-3 files contain color-managed data. This means that any tool that can read PDF/X-3 files can also read PDF/X-1a files. PDF/X-3 is a subset of PDF/X-2.

PDF/X-2: Unlike the previous two formats designed to support blind-feed exchanges, PDF/X-2 is protected for exchanging information; it is more of a dialogue between the sender and the receiver of the information. PDF/X-2 is very similar to PDF/X-3, but PDF/X-2 allows the use of externally referenced data.

3. PDF/E (suitable for multimedia file processing)


PDF/E is mainly aimed at the engineering technology industry and is used for interactive engineering file exchange, especially for some of the latest features used by the engineering technology community of PDF technology, including object-level metadata and 3D models.

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