digital signatures questions

Hello,
 
I have some questions about digital signatures. Thank you for your help.
 
Tools used: Livecycle designer 8 and reader 8.
 
1 - When I digital sign a pdf (livecycle form version 8) and save the file to disk the signature indicates that despite the signature is valid, the document has been changed since it was signed. However I didn´t changed de document... This always happens.
  
2 - When a document is digital sign what should be submited to the server application? All the form (pdf) or just the data (because a signature field is a data field, the signature would be submited right)? If I submit only the data, will I have the possibility to recreate the form with the digital signature?
 
3 - What is the point of using a data signature? Is just for the server application to validate data tampering? How can this affect the way and what data should be saved into the data file?
 
4 - Any good links where I can read about digital signing and PDF?
 
Thank you