PDFs may contain several types of data. These include text, raster graphics, vector graphics, and fonts (glyphs). A PDF file does not necessarily contain all the types of data and some PDF files are different from what they seem. For example, a PDF file might look like it has text in it while in fact, it is only displaying an image that contains the text. Text, displayed in this fashion, behaves differently than regular text when the file is manipulated by the display software.
Text in a PDF is referenced to a specific font and font size. This reference may be to fonts that are enclosed in the file itself or to external fonts that are expected to be available to the rendering software. If the fonts are not available, the output may not render properly. Internal fonts allow the characters to be available. This is discussed further in Section 5 Fonts.
Images (graphics) may be either raster or vector-based. A raster image (also known as bitmapped) is often created by scanning. The resolution of the image (pixel width and height) is determined when the file is created. The rendering software scales the picture based on information in the file. Vector-based images are built with lines and mathematical curves. These types of images can be zoomed in on without losing the quality of the image. When a PDF is created from scanned images, the PDF acts simply as a container for the image without any content text inside the file.
In this section, GlobalVision describes best practices when creating text within PDFs.