write text in existing pdf using c# and asp.net

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C#

ASP.NET

Hi All,

i have a pdf file which is some kind of form which contain application number field.i want to write or add auto generate number to this pdf. when ever user download this pdf it contain distinct number. for example if i download first time than application no generate 1 and add or write in pdf application no. field. if i download next time than it will write or add application number 2 in the pdf file.

please suggest me how can i implement this?
if possible than provide me some examples......