The question "Why GeoPDF?" has come up a lot lately primarily because Adobe released their GeoSpatial PDF format. I would like to share what I know about this format and possibly clear the air about the question at hand...
What is GeoPDF? GeoPDF is a geospatial extension to the Adobe PDF file format, from TerraGo Technologies. It is used to present GIS and mapping data in a standard Adobe Systems PDF. This extension adds a coordinate transformation matrix and other metadata to allow transformation of PDF coordinates to a projected Cartesian coordinate system. GeoPDFs often include other advanced PDF features such as layers and object data which can add significant GIS functionality to the file, particularly when used with the TerraGo Technologies plugin to Adobe Reader.
GeoPDF is still a PDF which means that ANYONE with Adobe Acrobat Reader can view your map data (look in the link to see that whoever added the Adobe Acrobat definition in Wikipedia, chose to display a graphic of a map!). It's true that you must download the GeoPDF Toolbar to gain access to the GIS functionality but wouldn't you expect to do this in most applications to access the intended functionality? I often write about Google Earth, Photosynth, and a variety of other GIS/Visualization Tools all of which require a download of some kind! A key benefit of the GeoPDF Toolbar is that you get GPS Tracking functionality.
What information can I include in my GeoPDF? There is quite a bit of data that can be added! For example, let's say that I am using Map2PDF for ArcGIS and in my map layout I have a raster image as a background, a shapefile for my streets data, an ArcSDE layer for parcel data and finally I create custom symbology and labels for each layer. I can export each one of these features to GeoPDF (with hyperlinks and annotations) in a separate layer in the final map. How is attribute data maintained? The GeoPDF export creates the equivalent for attribute data which is called Object Data. By creating object data in your output map, which is optional by the way, you can search for all the attribute data just as if it were in its original form in ArcGIS. With the ability to Zoom To a selected feature, you have a pretty powerful geo-search tool.
Map2PDF for Acrobat add-ons included GeoMark tools that allow users to add shapes and standard or custom stamps with comments. MapAssembler lets users combine maps and grids to create Map Books. Map2PDF for Acrobat lets users turn any PDF-based map into a georegistered map - a GeoPDF file - simply by entering two coordinates. But, there's much more to Map2PDF for Acrobat than that.
Map2PDF for Raster: I have seen 6 gigabytes of raster data compressed down to a 400 megabyte GeoPDF. Wow! That is an impressive compression ratio! I mentioned earlier that we can create layered PDFs which means that with our raster data, we can see a base layer as an RGB image but then we can add additional bands to be turned on and off as needed. For Landsat ETM+ data we can create a base map with bands 1, 2 and 3 then add additional bands and assign a particular color to each one. Because they are treated as seperate layers, you have the ability to turn them on and off dynamically. Imagine what you can do with low-resolution multispectral data like AVHRR? MODIS/ASTER? Click here to download some example data.