There appears to be an unresolved bug (as of writing this article) for users using Windows 10 and the free Adobe Reader in which only one PDF can be opened at a time.
Users can open the first PDF normally. However, once the Adobe Reader application is open a second PDF can not be opened. Oddly enough, double clicking on the PDF will add the PDF the to Recent Files within Windows File Explorer but it does not create a new tab or window within the Adobe Reader application.
This article provides a solution to the issue where you can not open multiple PDF’s within Adobe Reader.