Since most PDAs have built-in web-browsers, a thin-client solution might be a good idea. You would need to build a web site and database. Just open a browser on the PDA and open the site. The office would access a seperate part of the web application to view the submitted data. The nice thing about this solution is that it will work on any PDA with a browser and there will never be a need to install new software on the PDAs if new or different pieces of information need to be captured.
Almost any web scripting language, webserver, and database could be used. A very simple solution would be to not even run a database, but collect data using a form-mail web page which would submit data to an email address at the main office.
If an offline solution is desired (collecting the data of the course of a day, then submitting everything as a batch at the end of the day), it would be a little more complicated (in terms of development and maintenance) but you would have the added flexibility of being able to collect data when there is no Internet connection. In this case, you would want to have an application running on the PDA to collect and store the data, then submit it over the web to a web server which would collect data from all the terminals and store in a central database.
In either case, you would have to determine what kind of security is needed and implement an appropriate solution.