Hi Suzanne,
It may seem like a simple task to you, but it is not, especially not for Office. Microsoft spent a lot of time and money in litigation with Adobe.
However, you can download a free version of Acrobat Pro 8 that will do what you want. It is dated. It will install and run on Windows 10.
https://www.techworld.com/download/office-busin...
Note: This is a non-Microsoft website. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.
If you check online, you can likely find other utilities that will combine pdf's.
I hope this helps.