PDF facts for kids

For other uses, see PDF (disambiguation).

Quick facts for kids
Portable Document Format
Adobe PDF icon
Filename extension	`.pdf`
Internet media type	`application/pdf`, `application/x-pdf` `application/x-bzpdf` `application/x-gzpdf`
Type code	`PDF` (including a single trailing space)
Uniform Type Identifier (UTI)	com.adobe.pdf
Magic number	`%PDF`
Developed by	Adobe Inc. (1991–2008) ISO (2008–)
Initial release	June 15, 1993; 32 years ago (1993-06-15)
Latest release	2.0
Extended to	PDF/A, PDF/E, PDF/UA, PDF/VT, PDF/X
Standard	ISO 32000-2
Open format?	Yes

The Portable Document Format (PDF) is a special type of file format. It was created by a company called Adobe in 1992. Think of it like a digital paper that looks the same everywhere.

PDF files are designed to show documents exactly as they were made. This means the text, pictures, and how everything is laid out will look the same. It doesn't matter what computer, software, or operating system you use.

PDF files are based on something called PostScript. Each PDF file holds a full description of a document. This includes the text, fonts (the style of letters), vector graphics (pictures made of lines and shapes), and raster images (like photos). All this information is needed to display the document correctly.

The idea for PDF started with a project called "The Camelot Project" in 1991. In 2008, PDF became an official international standard called ISO 32000. This means it's now a widely accepted way to share documents.

PDF files can hold more than just text and simple pictures. They can have interactive parts like forms you can fill out. They can also include videos, 3D objects, and other types of data. PDF also allows for encryption (to keep files private) and digital signatures (to prove who sent a file).

The Story of PDF

The idea for PDF started in 1991. A co-founder of Adobe, John Warnock, wanted to create a simpler version of PostScript. PostScript was mainly used for printing. This new version, called Interchange PostScript (IPS), would be great for showing pages on any screen. It would work on any computer system.

Adobe made the PDF rules free for everyone in 1993. At first, PDF was mostly used for publishing. It had some competition from other file types.

PDF used to be owned by Adobe. But on July 1, 2008, it became an open standard. This means anyone can use it without paying fees to Adobe. The International Organization for Standardization (ISO) now manages the rules for PDF. Adobe also said that anyone can use their patents related to PDF for free.

Over the years, the PDF standard has been updated. The latest version, PDF 2.0 (ISO 32000-2:2020), was released in December 2020. It made some improvements and removed old parts. In April 2023, the PDF Association made the PDF 2.0 standard free to download.

How PDF Files Work

A PDF file often mixes different types of content. These include:

Text: Words and sentences.
Vector graphics: Drawings made of lines and shapes.
Raster graphics: Pictures like photos.
Other media: Like videos or sounds.

Newer PDF versions can also have links to other parts of the document or websites. They can also include forms and even small programs called JavaScript.

PDF uses three main ideas:

A simpler version of the PostScript language. This helps create the layout and graphics.
A system to include or replace fonts. This makes sure the text looks right on any computer.
A way to put all these parts into one file. This file can also be compressed to make it smaller.

PostScript and PDF

PostScript is like a special language that tells a printer how to draw an image. It can handle graphics and even has some programming features. PDF is a simpler version of PostScript. It keeps the graphics commands but removes the complex programming parts.

PostScript was made for printing. It would send commands to a printer, and the printer would draw a page. If there was a mistake, you had to fix it and send the whole job again. It wasn't easy to jump to different pages.

PDF is different. It's made for storing and showing documents on screens.

PDF files are simpler and safer because they don't have complex programming.
PDF has supported transparent graphics since version 1.4. PostScript does not.
In PDF, what happens on one page doesn't affect other pages. This makes it easier to view.
All the information needed to show the document is inside the file. This makes PDF files easy to share.

One downside is that PDF files can sometimes be larger. Also, PDF 1.6 and newer versions can even include interactive 3D drawings!

Inside a PDF File

A PDF file is made up of different parts, like building blocks. It starts with a header that tells you it's a PDF file and its version.

PDF files are organized using "objects." These objects can be:

True or false values.
Numbers (whole or decimal).
Text (called "strings").
Names (like labels).
Lists of objects (called "arrays").
Collections of objects with names (called "dictionaries").
Streams: These usually hold large amounts of data, like images, often compressed.

There's also an index table near the end of the file. This table helps your computer quickly find different parts of the PDF. This means you don't have to read the whole file to open a specific page. It also allows for small changes to be saved without rewriting the whole file.

At the very end of a PDF file, there's a footer. It tells the computer where the index table starts and marks the end of the file.

PDF files can be "linearized" or "optimized." This means they are set up so that you can start reading them in a web browser even before the whole file has downloaded. This makes them faster to open online.

PDF pages can be very large. Adobe Acrobat allows pages up to 15 million by 15 million inches! That's a huge area.

The maximum size of an Acrobat PDF page, superimposed on a map of Europe.

How Graphics and Text Look

PDF uses a special system to draw graphics. It's like a map that tells the computer where to put things on the page. You can scale, rotate, or tilt pictures.

Vector Graphics

Vector graphics in PDF are made with "paths." Paths are usually lines and smooth curves. They can also be made from the outlines of text. Paths can be drawn with a border (stroked) or filled with color. They can also be used to cut out parts of an image (clipping). You can use different colors or even patterns to fill shapes.

Raster Images

Raster images (like photos) in PDF are stored with a description and the image data itself. These images are often compressed to make the file smaller. PDF supports many ways to compress images, like JPEG (for photos) and PNG (for graphics with sharp lines).

Normally, all images are stored inside the PDF file. But some special PDF versions, like PDF/A and PDF/X, require images to be fully embedded.

Text

Text in PDF is made of "text elements." These tell the computer where to draw characters. The characters use a specific "encoding" and a chosen "font."

A font object in PDF describes a digital typeface (the design of the letters). It can either describe the font's features or include the actual font file. If the font file is included, it's called an "embedded font." This makes sure the text always looks the same, even if you don't have that font on your computer.

There are 14 special fonts, like Times and Helvetica, that are often available in most PDF readers. However, it's always best to embed fonts to be sure they display correctly.

If a document is scanned to PDF without optical character recognition (OCR), it's just an image. You can't select or search the text because the computer doesn't "know" it's text.

Transparency

Older PDF versions treated everything as solid. If you drew something, it completely covered what was underneath. But since PDF 1.4, you can use transparency. This means new objects can blend with what's already on the page. This creates cool effects, like those seen in programs like Adobe Illustrator.

Extra Features

PDF files can do much more than just show documents.

Making PDFs Accessible

A "tagged" PDF includes information about the document's structure. This helps people with disabilities, especially those using screen readers. It allows them to understand the order of content (text, graphics, images) and navigate the document easily.

An international standard called PDF/UA was created in 2012. It focuses specifically on making PDFs accessible.

Layers (Optional Content Groups)

Since PDF version 1.5, documents can have "layers," also called Optional Content Groups (OCGs). Think of these as different sheets of information that you can turn on or off. This is useful for things like maps, drawings, or documents in multiple languages. You can choose to see only the information you need.

Keeping Files Safe (Encryption and Signatures)

A PDF file can be encrypted to keep it private. You need a password to open or change it. PDF 2.0 uses strong 256-bit AES encryption.

PDF files can also have digital signatures. This is like a digital stamp that proves who sent the document and that it hasn't been changed.

Sometimes, PDF files have DRM rules. These rules try to stop people from copying, editing, or printing the document. But these rules depend on the software you use to open the PDF. So, they aren't always completely secure.

PDF security often uses two passwords:

A user password: This encrypts the file and stops people from opening it.
An owner password: This sets limits on what you can do with the document once it's open. For example, it might stop you from printing or copying text.

However, the owner password can often be removed easily. So, the restrictions it sets are not always guaranteed.

Attaching Files

PDF files can also have other files attached to them. You can open or save these attachments to your computer.

Document Information (Metadata)

PDF files can contain "metadata." This is information about the document itself. It can include the author, title, subject, and when it was created. This helps you organize and find documents. Newer PDF versions allow metadata to be attached to specific parts of the document, like images or fonts.

PDF documents can also store display settings. For example, they can tell your PDF viewer how to show the pages or what zoom level to use.

Multimedia in PDFs

"Rich Media PDF" means a PDF file that includes interactive content. This can be images, audio, video, or buttons. For example, a digital catalog could have product pictures with links to a website or buttons to order items directly.

Forms You Can Fill Out

PDF files can have "Interactive Forms." These are forms you can fill out directly on your computer. PDF supports two main types of forms:

AcroForms: These were introduced early on and allow text boxes, radio buttons, and some simple code.
XML Forms Architecture (XFA) forms: These were a different, more complex type of form. However, XFA forms are no longer part of the main PDF standard (PDF 2.0).

AcroForms allow you to submit the information you fill out to a website. They can also save your form data in separate files.

Who Owns PDF?

Anyone can create software that reads and writes PDF files. You don't have to pay money to Adobe Systems for this. Adobe owns patents related to PDF, but they allow others to use them for free if their software follows the PDF rules.

Security Concerns

PDF files can sometimes have security risks.

Changes to Content

Researchers have found ways to change the visible content in a signed PDF. This can happen without making the digital signature invalid. This means someone could change a document after it was signed, and it might still look legitimate.

Malware Risks

PDF files can sometimes carry viruses or other harmful software (malware). They might have hidden code that tries to exploit weaknesses in your PDF reader. This could happen if you open a malicious PDF file.

PDF attachments with viruses were first found in 2001. Since then, new weaknesses are found from time to time. This leads companies like Adobe to release security updates.

One way to protect yourself is to turn off JavaScript in your PDF reader. JavaScript is a programming language that can be used in PDFs. Disabling it can help prevent some attacks. Security experts say JavaScript isn't essential for viewing PDFs.

PDF Software

Many programs let you view PDF files for free. You can also buy software to change or edit PDF files.

Viewing and Editing

Many operating systems, like macOS and Linux, can create PDFs directly. Programs like LibreOffice and Microsoft Office can also save documents as PDFs. There are also "PDF printers" that let you "print" any document to a PDF file instead of a physical printer.

For those who use free software, there are libraries like Poppler. These help applications like Evince display PDFs. The Apache PDFBox project is another free tool for working with PDFs.

Printing PDFs

Special software called Raster image processors (RIPs) turn PDF files into a format that printers can understand. This allows high-quality printing on paper. Many professional printers accept PDF files for printing.

PDF is now a standard format for printing on many computer systems. This includes Common Unix Printing System and desktop programs like Firefox and LibreOffice. Some home printers can also print PDF files directly.

Native Display

PDF is the main "metafile" format for macOS. This means that the way graphics are drawn on a Mac is very similar to how they are described in a PDF. The Preview app on Mac can display PDF files, and the Safari browser can too. This system-level support means Mac apps can easily create PDF documents.

Adding Notes (Annotation)

Many programs let you add notes, highlight text, or draw on existing PDF files. Adobe Acrobat is a well-known example. Free software like Foxit Reader and Skim also offer these features. You can use these tools to mark up documents for studying or review.

Converting and Getting Information

PDF's main goal is to keep documents looking the same. But sometimes you need to convert a PDF to another format. Or you might want to pull out specific information like text, images, or tables. Many tools and software libraries can help with these tasks.