What do you mean by "digital signature file"? Digital Certificate you use to sign? If this is what you are looking for and you are using Acrobat/Reader XI, go to Edit->Preferences->Signatures->Identities&Trusted Certificates->More... In the left panel of the dialog that comes up expand Digital IDs, select Digital ID Files. The right pane will list all your digital ID files (.pfx or .p12) and folder where each of the is located.
I am curious why would you want to give your signing certificate to someone else. Digital IDs (signing certificates) are very personal and protected property. Why would you want to give your Digital ID to someone else? So that this person could sign documents pretending that you signed them? If you need to give your co-worker your certificate for purposes other than signing, you need to export your signing certificate (same dialog) with public key only (private key is not included) and give your co-worker the exported one.