How make a java program to sign and to give rights to save a pdf form?

Hi,
 
I would make a java program to sign a pdf and to allow users save its file offline.
Normally we have to use Reader Extension on server to do this. But in my case I wouldn't use a server.
Moreover I can't use Adobe Acrobat to give rights to save form because there is a restriction on the number of users of the form.
 
So how can I make my java code?
Can I use the API given with Reader Extension?
Are there examples or tutorials to show how to make it?
To sign a pdf, is the signature field obligatory ?
 
Thanks for your answer.
 
Best regards,