I read the first article previously. It states
"This is outside of your control - the headers being sent by the school's web server determine what file type Safari thinks it is, and therefore appends the extension it thinks is appropriate. In this case, most likely the Content-Type and/or Content-Disposition headers are not being sent, or not being sent correctly. See this for a more technical explanation."
It points here for the more technical explanation -
https://stackoverflow.com/questions/11487365/http-header-for-downloading-microsoft-word-and-excel-files
This involves code that I don't understand.
Also the pdfs in easysocial download with no problem - it is just the pdfs generated by payplans that seem to cause the issue. I think if it was a browser issue only it would affect all the pdfs.