Write some content in existing pdf file

Hi All,
i need some help on write some content in existing pdf file i.e auto generated application number on existing pdf file.

i have an application number field on my pdf file where i want to write an auto generated number in this field. Each time the pdf file is download the new application is written on the application number field. 


Please suggest me, i am using asp.net with c#.If possible any example or link, please provide.

Thanks