Portable Document Format (PDF) is a file format developed by Adobe in the 1990s to present documents, including text and images.
PDF is based on the PostScript language, which is a vector-based format to describe pages. In this sense it's similar to SVG, as it allows scaling a drawing to any size without losing its shape or amount of details.
Nevertheless, a PDF can contain many things, including text, fonts, vector graphics, and bitmap images. PDF was standardized as an open format in 2008, as ISO 32000.
PDF is commonly used as a sharing format, that is, to share documents, text, or images, that will display and print exactly as the author intended.
As FreeCAD is based on the Qt graphical toolkit, it can export the 3D view natively to PDF. This can be useful to export carefully created drawings that other users don't need to modify. For example, pages created with the TechDraw Workbench can be exported in this way for sharing or printing. In many cases it is also possible to perform two way conversion between PDF and SVG.
The option to export to PDF is available in the File → Export PDF menu.