I have a pdf (NOT A FORM) that I sent to someone to sign, it is an agreement between us. He signed and sent it back. I am trying to countersign, but I get the message 'the author of this form has specified the fields which you can fill. Click on any field to start filling form.'
BUT 1) THIS IS NOT A FORM!!!
2) I AM THE AUTHOR AND I CANNOT SIGN MY OWN PDF???
When you sign a file (form fields or not) it locks it, regardless of whether you created it or someone else.
The correct way of doing what you're trying to do is to add two signature fields in advance, and set only the second one to lock the file when signed. Then you send the file to the other person, have them sign the first field and send it back to you.
You then sign the second field and send them back the double-signed file.