PDF as the Portable Document Format is a file format that originates in printing and Adobe Postscript. Postscript is a programming language that can be used to describe the elements on a document to be able to print it in high quality ('elements' means here text and graphics).

The following illustration shows that a PDF file is also a “computer program” that describes how the content/elements of a page should look like. To display the content that is embedded in the PDF, the “code” has to be rendered, so that the text/images can be displayed on a screen or printed on paper. In this process the PDF code is rasterized to a pixel-based representation.

Structure of a Simple PDF

In the following simple example, you can see that a simple text file was converted to a PDF using a printer driver. To see the “programming code” of the PDF the extension “.pdf” was changed to “.txt” and then the file was opened with a text editor.
Technical Note: For complex PDFs, this approach is not working and not recommended, but for this case it is enough to illustrate how a PDF is built up internally.

The PDF code that describes the page of a document can include a lot of different elements, for example:

• Metadata
• Text (in various encoding)
• Vectors that look like text
• Vector-based drawings and
• Images
• Programming code
• Movies
• 3D / CAD data
• etc…

This enumeration already gives a rough idea how complicated the internal structure of a PDF can be. Technically it is also possible to put only a scanned document image into a PDF (= Image-only PDF). In this very simple case, no additional textual information is included.

Finally if any PDF Viewer displays a PDF on screen, it interprets the internal program code and composes a visual representation on screen, technically this process is called “PDF rendering”.

PDF Rendering/Opening and OCR