Can a recipient edit the text of a PDF that I send for signature?

Could anyone clarify whether a PDF is able to be edited by a client before they sign it?

I'm typically sending a document to 2 signatories - one external person who it goes to first, then one internal person at my company for countersignature.

It was my understanding that the document was not editable by any of the reciptients - other than any fields and signature fields assigned to them.

However, a colleague of mine has said that is a client has a Pro license themselves, then they can edit the document then sign it.  So they are telling me that I must do a comparison of the signed and original file before having it countersigned.

Is this true?  If it it, is there any way to prevent this?

Thanks.