You can use Web Service installed on your server, and this Web Service will be used by your PDA application for updating the database. Using this Web Service based application, your PDA application (written by me) will be able to synchronizing data, report, graphics, etc.
Also, using this method, you will have much experience in rapid development environment without leaving the quality of work, application stability, and security reason.
Last thing, all application will be written in .NET Framework. And it means I should know what PDA platform you will use within this project? Are they Pocket PC, Windows Mobile, or Windows CE?
Let me know,
Riyoyo