When English speaking people are talking about giving something or sending something to someone else it is common to use the word over, not just in the context you used but in lots of others as well. "Could you please pass it over to me." "Please send those papers over." "Hand it over." In the sentence you are asking about "I will fax them OVER" suggests that you are sending something to a particular person, either the person you are speaking to or the one which you have just been talking about. Saying "I will fax them" is just a general statement with no direction. It begs the question "to whom will you fax them?" The subject of the sentence has changed from the location to which the documents are going, to the documents themselves.

as in Lynx plus: adding "over" is short for "over there" or "over to you". when sending something from A to B we often say going over there ( over seas, meaning from North America to Europe) as opposed to just sending something (with no particular destination).