Combining PDFs loses the date and files are left unable to sort correct’y

I have 2+ PDFs, each with several email chains, sometimes into 3 digits. I need to sort them by date received and because they are emails, most have the same title. 
when I combine 2+ PDFs, all the dates disappear, I have a new modification date only, and 500 or so emails no longer sorted correctly. 
is there anyway to keep the original dste/time of each document within the pdf vs this new modification date? 
thank you.