This is the answer to my own questions that arose the other day when it appeared I was unable to modify the margins for the report I had created in MS Access 2007. I don't believe this solution is unique to 2007 and will resolve this issue regardless of the version of Access.
Basically the margin fields would not accept the changes I was attempting to input.
Solutions: I had set as my default printer to my pdf printer. As soon as I changed my default printer back to my HP, Viola - the fields accepted the changes.
I expect this is a rare occurrence (issue) given I was unable to find anyone else who posted this issue on a host of different forums. I elected to post my Issue/Resolution on this forum as it appears one of the most comprehensive for Q/A troubleshooting.