If you want to sign a PDF (e.g. E-billing), you need to order a certificate.
The easy, but expensive way is to order one from the official Adobe partners like these.
But you can also use a more affordable certificate like from A-Cert. However, this provider is not in the Adobe pre-installed certificate store by default. So you have to modify some settings (especially the receivers have to do this, if they want to check if the certificat is valid):
Preferences->Security->Advanced->Windows Integration->(Check all 3 boxes)
You can also do this via GPO / registry settings:
HKCUSoftwareAdobeAcrobat Reader10.0SecuritycPPKHandler -> bCertStoreImportEnable: DWORD, value 1
HKCUSoftwareAdobeAcrobat Reader10.0SecuritycASPKIcMSCAPI_DirectoryProvider -> iMSStoreTrusted: DWORD, value 62 (hex)