send and receive data on PDF via AJAX

I need to be able to send and receive data to and from PDF.

I've read at the chapter 15 of "Developing Acrobat Applications Using Javascript", you can establish exchange of information to and from PDF but I failed to see a sample app I can use.

let me tour you to the scenario:
1- I have pdf form (the type where there are a lot of information needed)
2- The client will fill up the form and submit it
3- I have a server-side script that catches the submitted form and save it to a database (MySQL, e.g.)
4- when the client will go back to the form again, he/she need not to complete the form again : he/she just need to input identifying data (username and password)
5- the PDF will send the username and password via HTTPrequest
6- the server-side script will do the query to provide the data and back to the client
7- upon receipt of the data, the PDF will do the filling of the rest of the fields
8- changes can be made as necessary and submit the form aka step 3

I need help on how I can implement from step 4. Can AJAX be used? Like so, it can be customized to solve the earlier posted problem.

Thanks in advance.