Is it possible to modify PDF document after Signed by *.p12 file?
Even author or anybody?
Is there any other way to sign PDF doncument such as device connected to my computer, galaxy phone/tab(Samsung), I-Phone/pad or etc.
Answer for Q1
Not exactly sure what you mean here, if you are referring to the "field locking" capability of a signature, then the answer is No, the locked fields cannot be modified (unless the signature is actually removed). Any fields that are not locked (not associated with the signature) can be filled in\changed.
If the document is dynamic, it can change after a signature is applied, but the signature "status" will report that changes have occurred.
Answer for Q2
Many different types of electronic signatures are supported for PDF (signature pads, biometric, digital signatures etc.,..) through plug-ins. I am not aware of any solutions that currently use iPads or phones to act as the "signing" device.
Check this link for information on security partners: http://www.adobe.com/security/partners/index.html