I will combine files into a single PDF and email to client. When the end user opens the PDF in Acrobat Reader, it displays at obscure preview size like 309%. The user then has to hit the minus key in the menu bar to see the PDF. Is there a setting when saving the PDF to have it open at 100% in reader?
Yes. Of course it depends on what you are using to combine PDFs.
In Acrobat choose
File > Properties and then select the Initial view tab.
Set it how you want things to open and save.
By default, PDFs are generally generated with the magnification set to "Fit to page" So the view is zoomed to allow the page to fit on the entire screen - that means enlarge or reduce as needed. So the "309%" is probably just the percentage needed to zoom the page to fit the client's screen.
Of Course, your client could always simply hit Command/Ctrl+1 to view