The signer's identity is unknown because it has not been included in the list of trusted identities and none or its parent certificates are trusted identities" indicates your recipients needs to add something to their copy of Acrobat/Reader. What the recipient of the document is missing is a certificate and only you can provide him with a copy of the certificate you created to sign the PDF. After you send him or make available a copy of the certificate, then he will need to import that certificate into his copy of Acrobat
If you want your PDF signature to be valid on your / co-worker's computer, you need to supply them with your Public certificate. To do this you need to export it out as an .fdf file, if in Acrobat X, choose Edit – Protection – Security Settings, select your ID that you used to sign the PDF with and choose Export, you can then either save it to disk, or directly email your certificate, get your co-worker to double click it in Adobe Reader and follow the prompt, when you see Set Contact Trust, select it and check Use Certificate as Trusted Root. Then click OK. Now open the PDF and it should now be valid.
You can also use Security Settings Export/Import feature in Acrobat XI Pro (Edit->Preferences->Security) to propagate trust from one installation to another.