I don't have an explanation why your different Acrobat installations behave differently. It may be due to preference settings.
A signature does not automatically prevent edits: What a signature allows you to do is to verify that a PDF document is identical to the version that was signed. Edits to the document will invalidate the signature. This means that you cannot just look at a document and determine that it was signed, and then assume that the version you are looking at is still the same version that was signed. You will need to verify the signature in order to be certain.
I am a little confused because from all the sources that I've read so far, a signature will prevent modifications. I've seen where it says "This document has been signed and can not be edited" and also where signatures are removed after the modification. Currently I am getting the "can not be edited" error and if I enter CTRL+D and look at the security tab, it says that "Changing the Document: Not Allowed".
My only confusion is how someone can bypass that since I've always at least seen that the signatures are removed(at the very least). I know you will have errors with those signatures even if you were to get to modify it, but since the documents are simple internal PO's, I don't think it would be an issue.
I'm checking preferences but no luck so far.
An option that is presented when applying one's digital signature is to lock the document, which prevents further editing. If a signer chose this option, the document would indeed prohibit edits. BUT, it's an option, not an automatically applied attribute. It is an acceptable workflow to receive a document, digitally sign it (approving the state of the doc as you see it at that moment), and pass it on for both further edits and signatures. The state of the document as you signed it is preserved within, even though there were more changes applied.
Signatures are not a no-edit security. Some software will ignore it anyway. It's more of a reminder. You seem to be very concerned about the possibility of editing but it's detectable and anyone properly trained to work with signatures will know it's been tampered with.
I too cannot edit a digitally signed pdf. I want to edit the document and retain the existing signatures and provide for additional signatures. Any ideas?
As mentioned, that is intentionally so. If you want to be able to edit the file after it is signed then don't use Digital Signatures.