Adobe® Acrobat® and Adobe Reader® version 9 support a new version of PDF that allows for encoding of map coordinate system and georeference information inside the PDF file.
[Update 2010 August 13: To make this work in ArcMap 9.3 you need to apply the ArcGIS (Desktop, Engine, Server) 9.3 Map Export Patch. That installs two DLLs that allow the Export Map Georeference Information checkbox. Without that, you're maps might have layers, but no coordinate system.]
This news release was in the summer 2008 issue (read) of the Arc user magazine but it came up last week at NACIS. Why is this better than other PDF solutions like TerraGo? It is completely free to both the cartographer and map reader and requires basic Adobe Acrobat (no plugins to download). ESRI and Adobe collaborated for a year on these PDF geospatial capabilities enhancements.
It works on both Mac and PC while other solutions are PC only. It is not full featured, though feature attributes and coordinates are accessible in Acrobat. But nifty tidbit: with Acrobat Pro you can import supplementary geo data into an existing geo PDF, even if it is in a different projection, and it will overlay onto the map. Might turn out to be a quick and dirty reprojection trick for those without ArcMap.
Most information below directly from ESRI promotional material.
ArcGIS 9.3 has new and improved functionality for exporting maps to Adobe PDF including map layers and location information for end users with Adobe Acrobat 9 or Reader 9 software. These enhancements are available as a freely downloadable patch for ArcGIS Desktop, ArcGIS Server, and ArcGIS Engine. This patch enables ArcGIS Desktop, Engine, and Server applications to export georeferenced PDF files.
When a georeferenced PDF is opened in a compatible viewer, such as Adobe Reader 9, the user can access geospatial functions like coordinate readout and find XY. This patch enables ArcGIS Desktop, Engine, and Server applications to export georeferenced PDF files.
Check out this video demonstration from ESRI TV: