Thanks for your reply Mark. How I wish I had this help over the weekend and not pulled out all my hair!!
I managed to get assistance with someone from technical support chat first thing Monday.
Just so that you know what resolved the problem:
1. Don't try to save a document in preflight when it's in a dropbox folder. It has to be saved to a desktop.
2. Under Document Properties the file was still showing the Mac Pages pdf version which apparently was conflicting with my settings. So I had to go to PRINT, print to PDF and, after the file had been converted this way, then go to Preflight and use STANDARDS. The file was fine after that, except it did not retain the correct page size, which luckily I caught in time, and changed that too before uploading the file.
The e-proof is supposed to arrive tomorrow. So far nothing wrong with the file. So all good at this point