Portable Document Format (PDF) is a file format developed by Adobe Systems for representing documents in a manner that is independent of the original application software, hardware, and operating system used to create those documents. A PDF file can describe documents containing any combination of text, graphics, and images in a device independent and resolution independent format. These documents can be one page or thousands of pages, very simple or extremely complex with a rich use of fonts, graphics, colour, and images.
PDF is a subset of those PostScript language elements that define the graphics, and only requires a very simple interpreter. For instance, flow control commands like if and loop are removed, while graphics commands such as lineto remain.
That means that the process of turning PDF back into a graphic is a matter of simply reading the description, rather than running a program in the PS interpreter. However the entire PS world in terms of fonts, layout and measurement remains intact.
Often the PostScript-like PDF code is generated from a source PostScript file. The graphics commands that the PS code outputs are collected and tokenized, any files, graphics or fonts the document references are also collected, and then everything is compressed into a single file.
PDF and HTML are not equivalent technologies, but are both commonly found on the Web.
HTML is a method for describing the content of a webpage in a manner that is open to interpretation by the browser which renders it on the user's screen. This permits content to be rendered to suit the viewer rather than the content provider. PDF on the other hand is strictly concerned with describing the content of a document such that the original intention of its author is fully preserved.
Since many content providers do not like the fluid nature of HTML rendering, PDF has become widespread to force a particular layout. With HTML the same can be achieved by using a raster graphics image to present text, but then the text can not be copied as such, nor can a subtext be searched within it. Use of images also leads to larger file sizes. (Sometimes the same is done in a PDF file, and the same disadvantages apply.)
A typical example of the differences this leads to for the viewer is with zooming:
Enlarging a PDF document magnifies the text but preserves the original layout and spacing; a practical limit on zooming follows from the requirement to keep a text column within the width of the screen (otherwise horizontal scrolling would be needed during and after reading each line, which would be very cumbersome).
With HTML a larger font size is used and lines re-wrap accordingly to fit the browser window.
vector graphics for coastlines, lakes, rivers, highways, markings of cities, and Interstate highway symbols - on zooming in, the curves remain sharp, they do not appear as consisting of enlarged pixels (i.e. rectangles of pixels)
text stored as such - ditto, and also one can copy the text
raster graphics for showing mountain relief - on zooming in, this consists of enlarged pixels (the blue of the sea and lakes is "filled" neatly to the vector graphics coast line, hence not part of this raster graphics layer)
An example of a PDF map without raster graphics is . In the map  the blue of the sea is not "filled" neatly to the vector graphics coast line, but just part of the raster graphics layer, giving a cruder result (noticeable when highly zoomed in).