Look for PFX files on your system. On a Windows XP/Acrobat 9 system of mine, it will stored a self-signed certificate here: C:\Documents and Settings\[username]\Application Data\Adobe\Acrobat\9.0\Security\username.pfx
If you can't find it, what version of Acrobat are you using?
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thanks George but i can not locate that file on my old computer. i am using acrobat x pro. please help.
Then it is likely that the certificate that you are looking for is in the Windows Certificate Store. You need to export it using Internet Explorer or the Certificate manager.